What does the following formalism mean?

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Hi, could you please explain to me what does the following formalisms mean?

View attachment 2414

i'm interested in the body part.

Thank you in advance.

 

[Evgeny.Makarov]

Hi, and welcome to MHB!

It looks like a formal definition of some programming language. This part shows what a method, or a function, is. The body may contain an assignment of a variable to a variable or of the result of a method call to a variable. The other two options I did not really understand; more context is needed.