signed_value =: monad define
  digits =. # y.
   if. 1 = 1 take y.
    do. ((digits copy 2) base y. ) - 2^digits
    else. (digits copy 2) base y.
   end.
)

sign_extend=: dyad define
  digits =. # y.
  ((0 >. x. - digits) copy 1 take y.) , y.