Take care, FORALL semantics are different to DO-loop semantics, Although a FORALL may look a bit like a DO loop it is more akin to an array assignment statement. A DO loop has an implied sequential ordering, a FORALL statement in completed in one go!
DO i = 2, n-1
a(i) = a(i-1) + a(i) + a(i+1)
END DO
is different to,
FORALL (i=2:n-1) &
a(i) = a(i-1) + a(i) + a(i+1)
which is the same as,
a(2:n-1) = a(1:n-2) + a(2:n-1) + a(3:n)