reads an elseif (expr): statements
function() {
    const result = this.node("if");
    let alternate = null;
    let test = null;
    let body = null;
    const items = [];
    test = this.next().read_if_expr();
    if (this.expect(":")) this.next();
    body = this.node("block");
    while (this.token != this.EOF && this.token !== this.tok.T_ENDIF) {
      if (this.token === this.tok.T_ELSEIF) {
        alternate = this.read_elseif_short();
        break;
      } else if (this.token === this.tok.T_ELSE) {
        alternate = this.read_else_short();
        break;
      }
      items.push(this.read_inner_statement());
    }
    body = body(null, items);
    return result(test, body, alternate, true);
  }