function command( repl ) {
	return onCommand;

	/**
	* Adds user-defined documentation.
	*
	* ## Notes
	*
	* -   If user-defined documentation already exists for a provided alias, the current documentation is overwritten.
	*
	* @private
	* @param {string} alias - alias
	* @param {*} [ref] - object reference
	* @param {string} doc - documentation
	* @returns {void}
	*/
	function onCommand( alias, ref, doc ) {
		var buf;
		var err;
		var d;
		var r;
		var i;
		if ( !isString( alias ) ) {
			err = new TypeError( format( 'invalid argument. First argument must be a string. Value: `%s`.', alias ) );
			debug( 'Error: %s', err.message );
			repl._ostream.write( 'Error: '+err.message+'\n' );
			return;
		}
		if ( arguments.length < 3 ) {
			d = ref;
		} else {
			r = ref;
			d = doc;
		}
		if ( !isString( d ) ) {
			err = new TypeError( format( 'invalid argument. Documentation argument must be a string. Value: `%s`.', d ) );
			debug( 'Error: %s', err.message );
			repl._ostream.write( 'Error: '+err.message+'\n' );
			return;
		}
		buf = repl._userdocs;

		// TODO: extract `info`, examples, etc

		// Search for existing documentation to overwrite...
		for ( i = 0; i < buf.length; i += 3 ) {
			if ( buf[ i ] === alias ) {
				buf[ i+1 ] = r;
				buf[ i+2 ] = {
					'text': d
				};
				return;
			}
		}
		// Append new documentation:
		buf.push( alias, r, {
			'text': d
		});
	}
}
Returns a callback to be invoked upon calling the `userDoc` command.

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