code: public static boolean hasSingleCharacterArg(Closure c) {
          if (c.getMaximumNumberOfParameters() != 1) return false;
          String typeName = c.getParameterTypes()[0].getName();
          return typeName.equals("char") || typeName.equals("java.lang.Character");
      }
docs: Does the Closure have a single char-like (char or Character) argument.
  @param c a Closure
  @return true if it has exactly one argument and the type is char or Character