module module_act
implicit none

contains

integer function act(f, a, b)
implicit none
integer, intent(in) :: a, b
interface
    integer function f(x, y)
    integer, intent(in) :: x, y
    end function f
end interface

    act = f(a, b)

end function act

end module module_act

program pr
use module_act
implicit none
interface
    integer function add(a, b)
    integer, intent(in) :: a, b
    end function add
end interface

    print *, "res = ", act(add, 1, 2)

end program pr

integer function add(a, b)
implicit none
integer, intent(in) :: a, b

    add = a + b

end function add

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