diff --git a/Cargo.toml b/Cargo.toml
index 99da6522..6d48bc8d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,7 +21,7 @@ num-traits = { version = "0.2.11", default-features = false }
 
 [features]
 #default = ["unchecked", "sync", "no_optimize", "no_float", "only_i32", "no_index", "no_object", "no_function", "no_module"]
-default = ["internals"]
+default = []
 plugins = []
 unchecked = []      # unchecked arithmetic
 sync = []           # restrict to only types that implement Send + Sync
diff --git a/doc/src/engine/custom-syntax.md b/doc/src/engine/custom-syntax.md
index 542efaa5..e0529841 100644
--- a/doc/src/engine/custom-syntax.md
+++ b/doc/src/engine/custom-syntax.md
@@ -1,5 +1,5 @@
-Extending Rhai with Custom Syntax
-================================
+Extend Rhai with Custom Syntax
+=============================
 
 {{#include ../links.md}}
 
@@ -131,7 +131,7 @@ Fn(
     state: &mut State,
     lib: &Module,
     this_ptr: &mut Option<&mut Dynamic>,
-    inputs: &[Expr],
+    inputs: &[Expression],
     level: usize
 ) -> Result<Dynamic, Box<EvalAltResult>>
 ```
@@ -144,7 +144,7 @@ where:
 * `state : &mut State` - mutable reference to the current evaluation state; **do not touch**.
 * `lib : &Module` - reference to the current collection of script-defined functions.
 * `this_ptr : &mut Option<&mut Dynamic>` - mutable reference to the current binding of the `this` pointer; **do not touch**.
-* `inputs : &[Expr]` - a list of input expression trees.
+* `inputs : &[Expression]` - a list of input expression trees.
 * `level : usize` - the current function call level.
 
 There are a lot of parameters, most of which should not be touched or Bad Things Happen™.
@@ -158,11 +158,11 @@ and statement blocks (`$block$) are provided.
 
 To access a particular argument, use the following patterns:
 
-| Argument type | Pattern (`n` = slot in `inputs`)         | Result type | Description        |
-| :-----------: | ---------------------------------------- | :---------: | ------------------ |
-|   `$ident$`   | `inputs[n].get_variable_name().unwrap()` |   `&str`    | name of a variable |
-|   `$expr$`    | `inputs.get(n).unwrap()`                 |   `Expr`    | an expression tree |
-|   `$block$`   | `inputs.get(n).unwrap()`                 |   `Expr`    | an expression tree |
+| Argument type | Pattern (`n` = slot in `inputs`)         | Result type  | Description        |
+| :-----------: | ---------------------------------------- | :----------: | ------------------ |
+|   `$ident$`   | `inputs[n].get_variable_name().unwrap()` |    `&str`    | name of a variable |
+|   `$expr$`    | `inputs.get(n).unwrap()`                 | `Expression` | an expression tree |
+|   `$block$`   | `inputs.get(n).unwrap()`                 | `Expression` | an expression tree |
 
 ### Evaluate an Expression Tree
 
@@ -213,7 +213,7 @@ fn implementation_func(
     state: &mut State,
     lib: &Module,
     this_ptr: &mut Option<&mut Dynamic>,
-    inputs: &[Expr],
+    inputs: &[Expression],
     level: usize
 ) -> Result<Dynamic, Box<EvalAltResult>> {
     let var_name = inputs[0].get_variable_name().unwrap().to_string();
diff --git a/doc/src/engine/dsl.md b/doc/src/engine/dsl.md
index 5534f243..3585936b 100644
--- a/doc/src/engine/dsl.md
+++ b/doc/src/engine/dsl.md
@@ -76,9 +76,9 @@ For its evaluation, the callback function will receive the following list of par
 
 `exprs[0] = "sum"` - math operator  
 `exprs[1] = "price"` - field name  
-`exprs[2] = Expr(table)` - data source  
+`exprs[2] = Expression(table)` - data source  
 `exprs[3] = "row"` - loop variable name  
-`exprs[4] = Expr(row.wright > 50)` - expression  
+`exprs[4] = Expression(row.wright > 50)` - expression  
 
 The other identified, such as `"select"`, `"from"`, as as as symbols `->` and `:` are
 parsed in the order defined within the custom syntax.
diff --git a/src/engine.rs b/src/engine.rs
index dfded485..0455c6f1 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -95,6 +95,36 @@ pub const MARKER_BLOCK: &str = "$block$";
 #[cfg(feature = "internals")]
 pub const MARKER_IDENT: &str = "$ident$";
 
+#[cfg(feature = "internals")]
+pub struct Expression<'a>(&'a Expr);
+
+#[cfg(feature = "internals")]
+impl<'a> From<&'a Expr> for Expression<'a> {
+    fn from(expr: &'a Expr) -> Self {
+        Self(expr)
+    }
+}
+
+#[cfg(feature = "internals")]
+impl Expression<'_> {
+    /// If this expression is a variable name, return it.  Otherwise `None`.
+    #[cfg(feature = "internals")]
+    pub fn get_variable_name(&self) -> Option<&str> {
+        match self.0 {
+            Expr::Variable(x) => Some((x.0).0.as_str()),
+            _ => None,
+        }
+    }
+    /// Get the expression.
+    pub(crate) fn expr(&self) -> &Expr {
+        &self.0
+    }
+    /// Get the position of this expression.
+    pub fn position(&self) -> Position {
+        self.0.position()
+    }
+}
+
 /// A type specifying the method of chaining.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
 enum ChainType {
@@ -1691,10 +1721,10 @@ impl Engine {
         state: &mut State,
         lib: &Module,
         this_ptr: &mut Option<&mut Dynamic>,
-        expr: &Expr,
+        expr: &Expression,
         level: usize,
     ) -> Result<Dynamic, Box<EvalAltResult>> {
-        self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)
+        self.eval_expr(scope, mods, state, lib, this_ptr, expr.expr(), level)
     }
 
     /// Evaluate an expression
@@ -2131,8 +2161,8 @@ impl Engine {
             #[cfg(feature = "internals")]
             Expr::Custom(x) => {
                 let func = (x.0).1.as_ref();
-                let exprs = (x.0).0.as_ref();
-                func(self, scope, mods, state, lib, this_ptr, exprs, level)
+                let ep: StaticVec<_> = (x.0).0.iter().map(|e| e.into()).collect();
+                func(self, scope, mods, state, lib, this_ptr, ep.as_ref(), level)
             }
 
             _ => unreachable!(),
diff --git a/src/lib.rs b/src/lib.rs
index 6e319260..a3edc8ca 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -169,7 +169,7 @@ pub use parser::{CustomExpr, Expr, ReturnType, ScriptFnDef, Stmt};
 
 #[cfg(feature = "internals")]
 #[deprecated(note = "this type is volatile and may change")]
-pub use engine::{Imports, State as EvalState};
+pub use engine::{Expression, Imports, State as EvalState};
 
 #[cfg(feature = "internals")]
 #[deprecated(note = "this type is volatile and may change")]
diff --git a/src/module.rs b/src/module.rs
index 7181e4d7..94bd40a4 100644
--- a/src/module.rs
+++ b/src/module.rs
@@ -342,7 +342,11 @@ impl Module {
     /// Set a Rust function into the module, returning a hash key.
     ///
     /// If there is an existing Rust function of the same hash, it is replaced.
-    pub(crate) fn set_fn(
+    ///
+    /// ## WARNING - Low Level API
+    ///
+    /// This function is very low level.
+    pub fn set_fn(
         &mut self,
         name: impl Into<String>,
         access: FnAccess,
diff --git a/src/parser.rs b/src/parser.rs
index 4df5b799..55fbdabf 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -908,14 +908,6 @@ impl Expr {
             _ => self,
         }
     }
-
-    #[cfg(feature = "internals")]
-    pub fn get_variable_name(&self) -> Option<&str> {
-        match self {
-            Self::Variable(x) => Some((x.0).0.as_str()),
-            _ => None,
-        }
-    }
 }
 
 /// Consume a particular token, checking that it is the expected one.
diff --git a/src/syntax.rs b/src/syntax.rs
index 2fea999e..05ab5e0b 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -2,11 +2,10 @@
 #![cfg(feature = "internals")]
 
 use crate::any::Dynamic;
-use crate::engine::{Engine, Imports, State, MARKER_BLOCK, MARKER_EXPR, MARKER_IDENT};
+use crate::engine::{Engine, Expression, Imports, State, MARKER_BLOCK, MARKER_EXPR, MARKER_IDENT};
 use crate::error::LexError;
 use crate::fn_native::{SendSync, Shared};
 use crate::module::Module;
-use crate::parser::Expr;
 use crate::result::EvalAltResult;
 use crate::scope::Scope;
 use crate::token::{is_valid_identifier, Token};
@@ -28,7 +27,7 @@ pub type FnCustomSyntaxEval = dyn Fn(
     &mut State,
     &Module,
     &mut Option<&mut Dynamic>,
-    &[Expr],
+    &[Expression],
     usize,
 ) -> Result<Dynamic, Box<EvalAltResult>>;
 /// A general function trail object.
@@ -71,7 +70,7 @@ impl Engine {
                 &mut State,
                 &Module,
                 &mut Option<&mut Dynamic>,
-                &[Expr],
+                &[Expression],
                 usize,
             ) -> Result<Dynamic, Box<EvalAltResult>>
             + SendSync
diff --git a/tests/syntax.rs b/tests/syntax.rs
index 501c90b3..0fdf3c07 100644
--- a/tests/syntax.rs
+++ b/tests/syntax.rs
@@ -1,6 +1,6 @@
 #![cfg(feature = "internals")]
 use rhai::{
-    Dynamic, Engine, EvalAltResult, EvalState, Expr, Imports, LexError, Module, Scope, INT,
+    Dynamic, Engine, EvalAltResult, EvalState, Expression, Imports, LexError, Module, Scope, INT,
 };
 
 #[test]
@@ -24,7 +24,7 @@ fn test_custom_syntax() -> Result<(), Box<EvalAltResult>> {
              state: &mut EvalState,
              lib: &Module,
              this_ptr: &mut Option<&mut Dynamic>,
-             inputs: &[Expr],
+             inputs: &[Expression],
              level: usize| {
                 let var_name = inputs[0].get_variable_name().unwrap().to_string();
                 let stmt = inputs.get(1).unwrap();