code: function add$1 ( keypath, value ) {
    	if ( value === undefined ) value = 1;
    	if ( !isNumeric( value ) ) throw new Error( 'Bad arguments' );
    	return set( this.ractive, build$1( this, keypath, value ).map( function ( pair ) {
    		var model = pair[0], val = pair[1], value = model.get();
    		if ( !isNumeric( val ) || !isNumeric( value ) ) throw new Error( 'Cannot add non-numeric value' );
    		return [ model, value + val ];
    	}) );
    }
docs: the usual mutation suspects