The Function's Effect
Now we describe how the function changes the stack and produces a value.
fPOP : [e0,e1,...,en-1] |? ( en-1, [e0,e1,...,en-2] )
The symbol " |? " means "maps to". On its left side is a description of a generic domain element. On its right side is a description of the corresponding range element.
This formula indicates that the POP operation takes an n-element stack, returns the last element, and removes it from the stack.