function command( repl ) {
	return onCommand;

	/**
	* Returns a boolean indicating whether a string is a reserved keyword in the REPL environment.
	*
	* ## Notes
	*
	* -   This function supports nested keyword paths (e.g., `base.sin`).
	*
	* @private
	* @param {string} keyword - string to test
	* @returns {(boolean|void)} boolean indicating whether a string is a reserved keyword
	*/
	function onCommand( keyword ) {
		var desc;
		var err;
		var o;
		var i;
		var k;
		if ( !isString( keyword ) ) {
			err = new TypeError( format( 'invalid argument. Must provide a string. Value: `%s`.', keyword ) );
			debug( 'Error: %s', err.message );
			repl._ostream.write( 'Error: '+err.message+'\n' );
			return;
		}
		// Our proxy for determining whether a value is a keyword is whether the value is read-only property on the `context` object, as any properties we have added to the `context` object are read-only (note, however, that, in a non-sandboxed environment, the `global` object may have read-only properties we have *not* introduced; meaning, some values which may be flagged as "keywords" were not introduced by us):
		keyword = keyword.split( '.' );
		o = repl._context;
		for ( i = 0; i < keyword.length-1; i++ ) {
			k = keyword[ i ];
			if ( !hasOwnProp( o, k ) ) {
				return false;
			}
			o = o[ k ];
		}
		k = keyword[ i ];
		desc = propertyDescriptor( o, k );
		return (
			desc !== null &&
			(
				// Data descriptor:
				desc.writable === false ||

				// Accessor descriptor:
				(
					typeof desc.get === 'function' &&
					desc.set === void 0
				)
			)
		);
	}
}
Returns a callback to be invoked upon calling the `isKeyword` command.

@private
@param {REPL} repl - REPL instance
@returns {Function} callback