FORTRAN has the convention that all parameter passing is by reference. Nevertheless, as described in the chapter, it is possible to call subroutines with nonvariable expressions as arguments, as in CALL P(X,X+Y,2).

(a) Explain how one can pass a variable X by value in FORTRAN.

(b) Suppose that subroutine P is declared as follows:

SUBROUTINE P(A)

INTEGER A PRINT *, A

A = A + 1 RETURN END

and is called from the main program as follows:

CALL P(1)

In some FORTRAN systems, this will cause a runtime error. In others, no runtime error occurs, but if the subroutine is called again with 1 as its argument, it may print the value 2. Explain how both behaviors might occur.