code: public char nextClean(char c) {
          char n = nextClean();
          if (n != c) {
              throw syntaxError("Expected '" + c + "' and instead saw '" + n + "'");
          }
          return n;
      }
docs: Consume the next character, skipping whitespace, and check that it matches a
  specified character.
  @param c The character to match.
  @return The character.