code: private Expression compileOperation(Operation operation, int opPos)
            throws TransformerException
    {
  
      int leftPos = getFirstChildPos(opPos);
      int rightPos = getNextOpPos(leftPos);
  
      operation.setLeftRight(compile(leftPos), compile(rightPos));
  
      return operation;
    }
docs: Bottle-neck compilation of an operation with left and right operands.
  
  @param operation non-null reference to parent operation.
  @param opPos The op map position of the parent operation.
  
  @return reference to {@link org.apache.xpath.operations.Operation} instance.
  
  @throws TransformerException if there is a syntax or other error.