diff --git a/Pantograph.lean b/Pantograph.lean
index 1e3dbe3..9d9399d 100644
--- a/Pantograph.lean
+++ b/Pantograph.lean
@@ -1,7 +1,7 @@
 import Pantograph.Goal
 import Pantograph.Protocol
 import Pantograph.Serial
-import Pantograph.Symbol
+import Pantograph.Environment
 import Lean.Data.HashMap
 
 namespace Pantograph
@@ -41,8 +41,9 @@ def execute (command: Protocol.Command): MainM Lean.Json := do
   | "reset"         => run reset
   | "stat"          => run stat
   | "expr.echo"     => run expr_echo
-  | "lib.catalog"   => run lib_catalog
-  | "lib.inspect"   => run lib_inspect
+  | "env.catalog"   => run env_catalog
+  | "env.inspect"   => run env_inspect
+  | "env.add"       => run env_add
   | "options.set"   => run options_set
   | "options.print" => run options_print
   | "goal.start"    => run goal_start
@@ -68,43 +69,20 @@ def execute (command: Protocol.Command): MainM Lean.Json := do
     let state ← get
     let nGoals := state.goalStates.size
     return .ok { nGoals }
-  lib_catalog (_: Protocol.LibCatalog): MainM (CR Protocol.LibCatalogResult) := do
-    let env ← Lean.MonadEnv.getEnv
-    let names := env.constants.fold (init := #[]) (λ acc name info =>
-      match to_filtered_symbol name info with
-      | .some x => acc.push x
-      | .none => acc)
-    return .ok { symbols := names }
-  lib_inspect (args: Protocol.LibInspect): MainM (CR Protocol.LibInspectResult) := do
+  env_catalog (args: Protocol.EnvCatalog): MainM (CR Protocol.EnvCatalogResult) := do
+    Environment.catalog args
+  env_inspect (args: Protocol.EnvInspect): MainM (CR Protocol.EnvInspectResult) := do
     let state ← get
-    let env ← Lean.MonadEnv.getEnv
-    let name :=  args.name.toName
-    let info? := env.find? name
-    match info? with
-    | none => return .error $ errorIndex s!"Symbol not found {args.name}"
-    | some info =>
-      let module? := env.getModuleIdxFor? name >>=
-        (λ idx => env.allImportedModuleNames.get? idx.toNat) |>.map toString
-      let value? := match args.value?, info with
-        | .some true, _ => info.value?
-        | .some false, _ => .none
-        | .none, .defnInfo _ => info.value?
-        | .none, _ => .none
-      return .ok {
-        type := ← (serialize_expression state.options info.type).run',
-        value? := ← value?.mapM (λ v => serialize_expression state.options v |>.run'),
-        publicName? := Lean.privateToUserName? name |>.map (·.toString),
-        -- BUG: Warning: getUsedConstants here will not include projections. This is a known bug.
-        typeDependency? := if args.dependency?.getD false then .some <| info.type.getUsedConstants.map (λ n => name_to_ast n) else .none,
-        valueDependency? := if args.dependency?.getD false then info.value?.map (·.getUsedConstants.map (λ n => name_to_ast n)) else .none,
-        module? := module?
-      }
+    Environment.inspect args state.options
+  env_add (args: Protocol.EnvAdd): MainM (CR Protocol.EnvAddResult) := do
+    Environment.addDecl args
   expr_echo (args: Protocol.ExprEcho): MainM (CR Protocol.ExprEchoResult) := do
     let state ← get
     let env ← Lean.MonadEnv.getEnv
-    match syntax_from_str env args.expr with
-    | .error str => return .error $ errorI "parsing" str
-    | .ok syn => runTermElabM do
+    let syn ← match syntax_from_str env args.expr with
+      | .error str => return .error $ errorI "parsing" str
+      | .ok syn => pure syn
+    runTermElabM (do
       match ← syntax_to_expr syn with
       | .error str => return .error $ errorI "elab" str
       | .ok expr => do
@@ -115,7 +93,7 @@ def execute (command: Protocol.Command): MainM Lean.Json := do
               expr := (← serialize_expression (options := state.options) expr)
           }
         catch exception =>
-          return .error $ errorI "typing" (← exception.toMessageData.toString)
+          return .error $ errorI "typing" (← exception.toMessageData.toString))
   options_set (args: Protocol.OptionsSet): MainM (CR Protocol.OptionsSetResult) := do
     let state ← get
     let options := state.options
diff --git a/Pantograph/Environment.lean b/Pantograph/Environment.lean
new file mode 100644
index 0000000..ab4513f
--- /dev/null
+++ b/Pantograph/Environment.lean
@@ -0,0 +1,104 @@
+import Pantograph.Protocol
+import Pantograph.Serial
+import Lean
+
+open Lean
+open Pantograph
+
+namespace Pantograph.Environment
+
+abbrev CR α := Except Protocol.InteractionError α
+
+def is_symbol_unsafe_or_internal (n: Lean.Name) (info: Lean.ConstantInfo): Bool :=
+  isLeanSymbol n ∨ (Lean.privateToUserName? n |>.map isLeanSymbol |>.getD false) ∨ info.isUnsafe
+  where
+  isLeanSymbol (name: Lean.Name): Bool := match name.getRoot with
+    | .str _ name => name == "Lean"
+    | _ => true
+
+def to_compact_symbol_name (n: Lean.Name) (info: Lean.ConstantInfo): String :=
+  let pref := match info with
+  | .axiomInfo  _ => "a"
+  | .defnInfo   _ => "d"
+  | .thmInfo    _ => "t"
+  | .opaqueInfo _ => "o"
+  | .quotInfo   _ => "q"
+  | .inductInfo _ => "i"
+  | .ctorInfo   _ => "c"
+  | .recInfo    _ => "r"
+  s!"{pref}{toString n}"
+
+def to_filtered_symbol (n: Lean.Name) (info: Lean.ConstantInfo): Option String :=
+  if is_symbol_unsafe_or_internal n info
+  then Option.none
+  else Option.some <| to_compact_symbol_name n info
+def catalog (_: Protocol.EnvCatalog): CoreM (CR Protocol.EnvCatalogResult) := do
+  let env ← Lean.MonadEnv.getEnv
+  let names := env.constants.fold (init := #[]) (λ acc name info =>
+    match to_filtered_symbol name info with
+    | .some x => acc.push x
+    | .none => acc)
+  return .ok { symbols := names }
+def inspect (args: Protocol.EnvInspect) (options: Protocol.Options): CoreM (CR Protocol.EnvInspectResult) := do
+  let env ← Lean.MonadEnv.getEnv
+  let name :=  args.name.toName
+  let info? := env.find? name
+  match info? with
+  | none => return .error $ Protocol.errorIndex s!"Symbol not found {args.name}"
+  | some info =>
+    let module? := env.getModuleIdxFor? name >>=
+      (λ idx => env.allImportedModuleNames.get? idx.toNat) |>.map toString
+    let value? := match args.value?, info with
+      | .some true, _ => info.value?
+      | .some false, _ => .none
+      | .none, .defnInfo _ => info.value?
+      | .none, _ => .none
+    return .ok {
+      type := ← (serialize_expression options info.type).run',
+      value? := ← value?.mapM (λ v => serialize_expression options v |>.run'),
+      publicName? := Lean.privateToUserName? name |>.map (·.toString),
+      -- BUG: Warning: getUsedConstants here will not include projections. This is a known bug.
+      typeDependency? := if args.dependency?.getD false then .some <| info.type.getUsedConstants.map (λ n => name_to_ast n) else .none,
+      valueDependency? := if args.dependency?.getD false then info.value?.map (·.getUsedConstants.map (λ n => name_to_ast n)) else .none,
+      module? := module?
+    }
+def addDecl (args: Protocol.EnvAdd): CoreM (CR Protocol.EnvAddResult) := do
+  let env ← Lean.MonadEnv.getEnv
+  let tvM: Elab.TermElabM (Except String (Expr × Expr)) := do
+    let type ← match syntax_from_str env args.type with
+      | .ok syn => do
+        match ← syntax_to_expr syn with
+        | .error e => return .error e
+        | .ok expr => pure expr
+      | .error e => return .error e
+    let value ← match syntax_from_str env args.value with
+      | .ok syn => do
+        try
+          let expr ← Elab.Term.elabTerm (stx := syn) (expectedType? := .some type)
+          Lean.Elab.Term.synthesizeSyntheticMVarsNoPostponing
+          let expr ← instantiateMVars expr
+          pure $ expr
+        catch ex => return .error (← ex.toMessageData.toString)
+      | .error e => return .error e
+    pure $ .ok (type, value)
+  let (type, value) ← match ← tvM.run' (ctx := {}) |>.run' with
+    | .ok t => pure t
+    | .error e => return .error $ Protocol.errorExpr e
+  let constant := Lean.Declaration.defnDecl <| Lean.mkDefinitionValEx
+    (name := args.name.toName)
+    (levelParams := [])
+    (type := type)
+    (value := value)
+    (hints := Lean.mkReducibilityHintsRegularEx 1)
+    (safety := Lean.DefinitionSafety.safe)
+    (all := [])
+  let env' ← match env.addDecl constant with
+    | .error e => do
+      let options ← Lean.MonadOptions.getOptions
+      let desc ← (e.toMessageData options).toString
+      return .error $ { error := "kernel", desc }
+    | .ok env' => pure env'
+  Lean.MonadEnv.modifyEnv (λ _ => env')
+  return .ok {}
+
+end Pantograph.Environment
diff --git a/Pantograph/Goal.lean b/Pantograph/Goal.lean
index 1589408..9b68319 100644
--- a/Pantograph/Goal.lean
+++ b/Pantograph/Goal.lean
@@ -1,7 +1,5 @@
 import Lean
 
-import Pantograph.Symbol
-
 def Lean.MessageLog.getErrorMessages (log : MessageLog) : MessageLog :=
   {
     msgs := log.msgs.filter fun m => match m.severity with | MessageSeverity.error => true | _ => false
diff --git a/Pantograph/Protocol.lean b/Pantograph/Protocol.lean
index a88a54a..c0204d0 100644
--- a/Pantograph/Protocol.lean
+++ b/Pantograph/Protocol.lean
@@ -79,6 +79,9 @@ structure InteractionError where
   desc: String
   deriving Lean.ToJson
 
+def errorIndex (desc: String): InteractionError := { error := "index", desc }
+def errorExpr (desc: String): InteractionError := { error := "expr", desc }
+
 
 --- Individual command and return types ---
 
@@ -102,13 +105,13 @@ structure ExprEchoResult where
   deriving Lean.ToJson
 
 -- Print all symbols in environment
-structure LibCatalog where
+structure EnvCatalog where
   deriving Lean.FromJson
-structure LibCatalogResult where
+structure EnvCatalogResult where
   symbols: Array String
   deriving Lean.ToJson
 -- Print the type of a symbol
-structure LibInspect where
+structure EnvInspect where
   name: String
   -- If true/false, show/hide the value expressions; By default definitions
   -- values are shown and theorem values are hidden.
@@ -116,7 +119,7 @@ structure LibInspect where
   -- If true, show the type and value dependencies
   dependency?: Option Bool := .some false
   deriving Lean.FromJson
-structure LibInspectResult where
+structure EnvInspectResult where
   type: Expression
   value?: Option Expression := .none
   module?: Option String    := .none
@@ -125,6 +128,14 @@ structure LibInspectResult where
   typeDependency?: Option (Array String) := .none
   valueDependency?: Option (Array String) := .none
   deriving Lean.ToJson
+structure EnvAdd where
+  name: String
+  type: String
+  value: String
+  isTheorem?: Bool
+  deriving Lean.FromJson
+structure EnvAddResult where
+  deriving Lean.ToJson
 
 /-- Set options; See `Options` struct above for meanings -/
 structure OptionsSet where
diff --git a/Pantograph/Symbol.lean b/Pantograph/Symbol.lean
deleted file mode 100644
index ba80877..0000000
--- a/Pantograph/Symbol.lean
+++ /dev/null
@@ -1,29 +0,0 @@
-import Lean
-
-namespace Pantograph
-
-def is_symbol_unsafe_or_internal (n: Lean.Name) (info: Lean.ConstantInfo): Bool :=
-  isLeanSymbol n ∨ (Lean.privateToUserName? n |>.map isLeanSymbol |>.getD false) ∨ info.isUnsafe
-  where
-  isLeanSymbol (name: Lean.Name): Bool := match name.getRoot with
-    | .str _ name => name == "Lean"
-    | _ => true
-
-def to_compact_symbol_name (n: Lean.Name) (info: Lean.ConstantInfo): String :=
-  let pref := match info with
-  | .axiomInfo  _ => "a"
-  | .defnInfo   _ => "d"
-  | .thmInfo    _ => "t"
-  | .opaqueInfo _ => "o"
-  | .quotInfo   _ => "q"
-  | .inductInfo _ => "i"
-  | .ctorInfo   _ => "c"
-  | .recInfo    _ => "r"
-  s!"{pref}{toString n}"
-
-def to_filtered_symbol (n: Lean.Name) (info: Lean.ConstantInfo): Option String :=
-  if is_symbol_unsafe_or_internal n info
-  then Option.none
-  else Option.some <| to_compact_symbol_name n info
-
-end Pantograph
diff --git a/README.md b/README.md
index 2b8425c..1ca4d7b 100644
--- a/README.md
+++ b/README.md
@@ -42,13 +42,13 @@ also accept lean options of the form `--key=value` e.g. `--pp.raw=true`.
 Example: (~5k symbols)
 ```
 $ pantograph Init
-lib.catalog
-lib.inspect {"name": "Nat.le_add_left"}
+env.catalog
+env.inspect {"name": "Nat.le_add_left"}
 ```
 Example with `mathlib4` (~90k symbols, may stack overflow, see troubleshooting)
 ```
 $ pantograph Mathlib.Analysis.Seminorm
-lib.catalog
+env.catalog
 ```
 Example proving a theorem: (alternatively use `goal.start {"copyFrom": "Nat.add_comm"}`) to prime the proof
 ```
@@ -68,8 +68,8 @@ where the application of `assumption` should lead to a failure.
 See `Pantograph/Protocol.lean` for a description of the parameters and return values in JSON.
 - `reset`: Delete all cached expressions and proof trees
 - `expr.echo {"expr": <expr>}`: Determine the type of an expression and round-trip it
-- `lib.catalog`: Display a list of all safe Lean symbols in the current context
-- `lib.inspect {"name": <name>, "value": <bool>}`: Show the type and package of a
+- `env.catalog`: Display a list of all safe Lean symbols in the current environment
+- `env.inspect {"name": <name>, "value": <bool>}`: Show the type and package of a
   given symbol; If value flag is set, the value is printed or hidden. By default
   only the values of definitions are printed.
 - `options.set { key: value, ... }`: Set one or more options (not Lean options; those
diff --git a/Test/Catalog.lean b/Test/Environment.lean
similarity index 73%
rename from Test/Catalog.lean
rename to Test/Environment.lean
index 44c2bf7..3bd8448 100644
--- a/Test/Catalog.lean
+++ b/Test/Environment.lean
@@ -1,8 +1,8 @@
 import LSpec
 import Pantograph.Serial
-import Pantograph.Symbol
+import Pantograph.Environment
 
-namespace Pantograph.Test.Catalog
+namespace Pantograph.Test.Environment
 
 open Pantograph
 open Lean
@@ -14,7 +14,7 @@ def test_symbol_visibility (env: Environment): IO LSpec.TestSeq := do
   ]
   let suite := entries.foldl (λ suites (symbol, target) =>
     let constant := env.constants.find! symbol
-    let test := LSpec.check symbol.toString ((is_symbol_unsafe_or_internal symbol constant) == target)
+    let test := LSpec.check symbol.toString ((Environment.is_symbol_unsafe_or_internal symbol constant) == target)
     LSpec.TestSeq.append suites test) LSpec.TestSeq.done
   return suite
 
@@ -24,7 +24,7 @@ def suite: IO LSpec.TestSeq := do
     (opts := {})
     (trustLevel := 1)
 
-  return LSpec.group "Catalog" $
+  return LSpec.group "Environment" $
     (LSpec.group "Symbol visibility" (← test_symbol_visibility env))
 
-end Pantograph.Test.Catalog
+end Pantograph.Test.Environment
diff --git a/Test/Integration.lean b/Test/Integration.lean
index 5aad1e7..0a6c210 100644
--- a/Test/Integration.lean
+++ b/Test/Integration.lean
@@ -44,20 +44,20 @@ def test_option_modify : IO LSpec.TestSeq :=
   let module? := Option.some "Init.Data.Nat.Basic"
   let options: Protocol.Options := {}
   subroutine_runner [
-    subroutine_step "lib.inspect"
+    subroutine_step "env.inspect"
       [("name", .str "Nat.add_one")]
      (Lean.toJson ({
        type := { pp? }, module? }:
-      Protocol.LibInspectResult)),
+      Protocol.EnvInspectResult)),
     subroutine_step "options.set"
       [("printExprAST", .bool true)]
      (Lean.toJson ({ }:
       Protocol.OptionsSetResult)),
-    subroutine_step "lib.inspect"
+    subroutine_step "env.inspect"
       [("name", .str "Nat.add_one")]
      (Lean.toJson ({
        type := { pp?, sexp? }, module? }:
-      Protocol.LibInspectResult)),
+      Protocol.EnvInspectResult)),
     subroutine_step "options.print"
       []
      (Lean.toJson ({ options with printExprAST := true }:
@@ -112,12 +112,49 @@ def test_tactic : IO LSpec.TestSeq :=
       Protocol.GoalTacticResult))
   ]
 
+def test_env : IO LSpec.TestSeq :=
+  let name1 := "Pantograph.mystery"
+  let name2 := "Pantograph.mystery2"
+  subroutine_runner [
+    subroutine_step "env.add"
+      [
+        ("name", .str name1),
+        ("type", .str "Prop → Prop → Prop"),
+        ("value", .str "λ (a b: Prop) => Or a b"),
+        ("isTheorem", .bool false)
+      ]
+     (Lean.toJson ({}: Protocol.EnvAddResult)),
+    subroutine_step "env.inspect"
+      [("name", .str name1)]
+     (Lean.toJson ({
+       value? := .some { pp? := .some "fun a b => a ∨ b" },
+       type := { pp? := .some "Prop → Prop → Prop" },
+     }:
+      Protocol.EnvInspectResult)),
+    subroutine_step "env.add"
+      [
+        ("name", .str name2),
+        ("type", .str "Nat → Int"),
+        ("value", .str "λ (a: Nat) => a + 1"),
+        ("isTheorem", .bool false)
+      ]
+     (Lean.toJson ({}: Protocol.EnvAddResult)),
+    subroutine_step "env.inspect"
+      [("name", .str name2)]
+     (Lean.toJson ({
+       value? := .some { pp? := .some "fun a => Int.ofNat a + 1" },
+       type := { pp? := .some "Nat → Int" },
+     }:
+      Protocol.EnvInspectResult))
+  ]
+
 def suite: IO LSpec.TestSeq := do
 
   return LSpec.group "Integration" $
     (LSpec.group "Option modify" (← test_option_modify)) ++
     (LSpec.group "Malformed command" (← test_malformed_command)) ++
-    (LSpec.group "Tactic" (← test_tactic))
+    (LSpec.group "Tactic" (← test_tactic)) ++
+    (LSpec.group "Env" (← test_env))
 
 
 end Pantograph.Test.Integration
diff --git a/Test/Main.lean b/Test/Main.lean
index 5178e85..4a8ab1f 100644
--- a/Test/Main.lean
+++ b/Test/Main.lean
@@ -1,5 +1,5 @@
 import LSpec
-import Test.Catalog
+import Test.Environment
 import Test.Holes
 import Test.Integration
 import Test.Proofs
@@ -15,7 +15,7 @@ def main := do
     Integration.suite,
     Proofs.suite,
     Serial.suite,
-    Catalog.suite
+    Environment.suite
   ]
   let all ← suites.foldlM (λ acc m => do pure $ acc ++ (← m)) LSpec.TestSeq.done
   LSpec.lspecIO $ all
diff --git a/Test/Serial.lean b/Test/Serial.lean
index 0730bad..e502fa8 100644
--- a/Test/Serial.lean
+++ b/Test/Serial.lean
@@ -1,6 +1,5 @@
 import LSpec
 import Pantograph.Serial
-import Pantograph.Symbol
 
 namespace Pantograph.Test.Serial