code: public Method resolveFunction(String prefix, String localName) {
          FunctionMapperSpi functionMapperSpi = findFunctionMapperSpi(prefix);
          return functionMapperSpi.resolveFunction(localName);
      }
docs: Resolves a Function via prefix and local name.
  @param prefix of the function
  @param localName of the function
  @return an instance of a Method