Structure       Val             From    To      Semantics
-------------------------------------------------------------------------------
Word            fromInt         int     word    lowbits or sign-extend
Word            fromIntZ        int     word    lowbits of zero-extend
Word            fromWord        word    word    lowbits or zero-extend
Word            fromWordX       word    word    lowbits of sign-extend
Word            toInt           word    int     overflow check, unsigned
Word            toIntX          word    int     overflow check, signed
Word            toWord          word    word    lowbits or zero-extend
Word            toWordX         word    word    lowbits or sign-extend

Int             fromInt         int     int     overflow check, signed
Int             fromWord        word    int     overflow check, unsigned
Int             fromWordX       word    int     overflow check, signed
Int             toInt           int     int     overflow check, signed
Int             toWord          int     word    lowbits or zero-extend
Int             toWordX         int     word    lowbits or sign-extend


From:           int, word
To:             int, word
Semantics:      lowbits or sign-extend,
                lowbits or zero-extend,
                overflow check, unsigned
                overflow check, signed


Primitives are all: lowbits or sign-extend, lowbits or zero-extend
