function stack() {
  this.elements = []
  this.index    = -1
  this.push = function(val) {
    this.elements[++this.index] = val
  }
  this.pop = function() {
    if (this.index < 0)
      return undefined
    return this.elements[this.index--]
  }
  return this
}

var _AcceptedSymbols = [
    // Language specifics
    '->', '$', '@', '~', '.', ',', ':', ';', '?', '\\'
    // Arithmethic
    '+', '-', '*', '/', '%', '^', '=',
    // Booleans
    '<=', '<', '==', '!=', '>', '>=', '&&', '||', '!',
    // Braces
    '[', ']', '{', '}', '(', ')',
    // Fractions and special notations
    "frac"
];