MEMO: (col-number-hash) ( -- hashtable )
    CHAR: a CHAR: h [a,b]
    1 8 [a,b] zip >hashtable ;  

: col>number ( letter -- number )
    (col-number-hash) at ;

:: valid-incline? ( move-x current-x move-y current-y -- ? )
    move-x current-x - :> dx
    move-y col>number current-y col>number - :> dy 
    dy dx = 
    dy dx -1 * =  or ;

:: inside-board? ( move-x move-y -- ? )
    8 [1,b] move-x move-y col>number 
    [ swap member? ] bi-curry@ bi and ;
    
:: valid-bishop? ( current-x current-y move-x move-y -- ? )
! not same location, member of a 1-8 range, and dy/dx = 1 or -1
    { 
    [ move-x current-x = not ]
    [ move-x move-y inside-board? ]
    [ move-x current-x move-y current-y valid-incline? ] 
    } 0&& ;