SIGNATURE BagMap[from ,< :from ** from -> bool, to, < :to ** to -> bool]

--
-- Parameter
SORT from to

FUN < : from ** from -> bool
FUN < : to ** to -> bool
        -- total strict-order; in 
        --   ~(x < y) AND ~(y < x) => x = y
        -- should = represent semantic equality

IMPORT Bag[from, <] ONLY bag
       Bag[to, <]   ONLY bag

FUN * : (from -> to)** bag[from, <] -> bag[to, <]                       
        -- f * s
        -- apply f to all elements of s