How to extend methods in e-language

Specman e-language has the built in features to extend the predefined and
user-defined methods in order to have new contents.
The extension can be done in the same file where the original method is declared or in another new file.
Specman e-language supports the following features for extending the methods

a) is first => which means add before the existing e code
b) is also => which means add after the existing e code
c) is only => which means override/replace the existing e code