function blockstmt(s, f) {
    var x = stmt(s, f);
    x.block = true;
    return x;
  }
Convenience function for defining block-statement-denoting symbols.

A block-statement-denoting symbol is one like 'if' or 'for', which will be
followed by a block and will not have to end with a semicolon.

@param {string} s - the name of the symbol
@param {function} - the first null denotation function for the symbol; see
                    the `expression` function for more detail

@returns {object} - the object describing the JSHint symbol (provided to
                    support cases where further refinement is necessary)