ziv@2240: functor OptionKeyFn(K : ORD_KEY) : ORD_KEY = struct
ziv@2240: 
ziv@2240: type ord_key = K.ord_key option
ziv@2240: 
ziv@2240: val compare =
ziv@2240:  fn (NONE, NONE) => EQUAL
ziv@2240:   | (NONE, _) => LESS
ziv@2240:   | (_, NONE) => GREATER
ziv@2240:   | (SOME x, SOME y) => K.compare (x, y)
ziv@2240: 
ziv@2240: end