implement main0 () = print"Hello World!\n" abstype basicprop (prop) absprop andprop (prop, prop) absprop trueprop datatype formula (prop) of | {x:prop} atom (x) of basicprop (x) | {x1,x2:prop} andopr (x1, x2) of (formula x1, formula x2) | True (trueprop)