coding is fine,but i did not understood how it works

[shireeshn]

reversing of string, output is comming correct.

FUNCTION rev RETURNS CHAR(INPUT ll AS CHAR).
IF LENGTH(ll) = 1 THEN
RETURN ll.
ELSE
DO:

RETURN rev(SUBSTRING(ll,2)) + SUBSTRING(ll,1,1).
END.
END.
DISP rev("shireesh").


can any one explain me how it works.