prebirth: Allow toplevel function applications

So we can invoke the main function for the program.

* build-aux/bootstrap/prebirth.js
  (Compiler#_cdfn): Handle non-`define' applications.
  (Compiler#assertApply): Remove function.

build-aux/bootstrap/prebirth.js

@@ -319,7 +319,18 @@ class Compiler
      */
     _cdfn( t )
     {
-        this.assertApply( t, 'define' );
+        // an application must be an s-expression
+        if ( !Array.isArray( t ) ) {
+            throw Error(
+                `\`${name}' application expected, found symbol \`${t.value}'`
+            );
+        }
+
+        // if it's not a definition, then it's a top-level application
+        if ( t[ 0 ].value !== 'define' )
+        {
+            return this._sexpToEs( t ) + ';';
+        }
 
         // e.g. (define (foo ...) body)
         const [ , [ { value: name }, ...params ], ...body ] = t;
@@ -441,34 +452,6 @@ class Compiler
         // final function application
         return `${idfn}(${argstr})`;
     }
-
-
-    /**
-     * Determine whether T is an application of a symbol NAME, or error
-     *
-     * @param {*}      t    hopefully a token or token list
-     * @param {string} name function name to assert against
-     */
-    assertApply( t, name )
-    {
-        // an application must be an s-expression
-        if ( !Array.isArray( t ) ) {
-            throw Error(
-                `\`${name}' application expected, found symbol \`${t.value}'`
-            );
-        }
-
-        // if there's a match, we can stop here
-        if ( t[ 0 ].value === name ) {
-            return;
-        }
-
-        // otherwise, provide an informative error of what we found and what
-        // we should have found
-        throw Error(
-            `\`${name}' expected, found \`${t[ 0 ].value}'`
-        );
-    }
 }