function startsWithConstant (expr, parenthesis) {
    let curNode = expr
    if (parenthesis === 'auto') {
      while (isParenthesisNode(curNode)) curNode = curNode.content
    }
    if (isConstantNode(curNode)) return true
    if (isOperatorNode(curNode)) {
      return startsWithConstant(curNode.args[0], parenthesis)
    }
    return false
  }
Returns true if the expression starts with a constant, under
the current parenthesization:
@param {Node} expression
@param {string} parenthesis
@return {boolean}