isPartialIdent = function (operator, next, previous) {
  if (operator === '-') {
    return next && previous && previous.is('interpolation');
  }
  return false;
}
Determine if operator is part an ident

@param {string} operator - The operator
@param {Object} next - The next node
@param {Object} previous - The previous node
@returns {bool} true / false