code: public boolean isFunction(final String variableExpression) {
          if (variableExpression == null || variableExpression.length() == 0) {
              return false;
          }
          
          for (int i = 0; i < functionLibraries.size(); i++) {
              FunctionLibrary lib = (FunctionLibrary)functionLibraries.get(i);
              if (variableExpression.startsWith(lib.getPrefix())) {
                  return true;
              }
          }
  
          return false;
      }
docs: Check if variable expression is a custom function.
  Expression has to start with one of the registered function library prefix.
  @param variableExpression to be checked
  @return flag (true/false)