Reads an entry
array_pair:
expr T_DOUBLE_ARROW expr
| expr
| expr T_DOUBLE_ARROW '&' variable
| '&' variable
| expr T_DOUBLE_ARROW T_LIST '(' array_pair_list ')'
| T_LIST '(' array_pair_list ')'
function(shortForm) {
    if (
      this.token === "," ||
      (!shortForm && this.token === ")") ||
      (shortForm && this.token === "]")
    ) {
      return null;
    }
    if (this.token === "&") {
      return this.next().read_variable(true, false, true);
    } else {
      const entry = this.node(ArrayEntry);
      const expr = this.read_expr();
      if (this.token === this.tok.T_DOUBLE_ARROW) {
        if (this.next().token === "&") {
          return entry(expr, this.next().read_variable(true, false, true));
        } else {
          return entry(expr, this.read_expr());
        }
      }
      return expr;
    }
  }