isNegativeNumber = function (operator, next, previous, doublePrevious) {
  if (operator === '-') {

    // Catch the following:
    // $foo: -20;
    // $foo: -#{$foo}
    // $foo: -($foo * 2)
    // $foo: -$foo
    if (next) {
      if (!previous || (previous.is('space') && doublePrevious && !doublePrevious.is('number'))) {
        if (
          next.is('number') ||
          next.is('interpolation') ||
          next.is('variable') ||
          next.is('parentheses')
        ) {
          return true;
        }
      }
    }

    // Catch the following:
    // .foo {
    //   property: -16px;
    // }
    if (next && (next.is('dimension') || next.is('percentage'))) {
      return true;
    }

    // Catch the following:
    // .foo {
    //   propery: 2 / -16;
    // }
    if (doublePrevious && doublePrevious.is('operator')) {
      return true;
    }

    // Catch the following:
    // .foo {
    //   property: 2 /-16px;
    // }
    if (previous && previous.is('operator')) {
      return true;
    }
  }

  return false;
}
Determine if operator is negative number

@param {string} operator - The operator
@param {Object} next - The next node
@param {Object} previous - The previous node
@param {Object} doublePrevious - The double previous node (back two)
@returns {bool} true / false