How can we differ that which statement is part of user defined function and which is a predefined function statement?

Answer 1

If you use a function without it's defination, it's a pre-defined function...
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If you define a function and use it... It's an user-defined function like the name suggests..
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And if you use name of pre-defined function for an user-defined function..
That user-defined function gets temporarily modified the entire session...