( r c -- r f )
( Parse digit c and accumulate into result r.
  Flag f is 0 when c was a valid digit, 1 when c was WS,
  -1 when c was an invalid digit. )
: _pdacc
    DUP 0x21 < IF DROP 1 EXIT THEN
    ( parse char )
    '0' -
    ( if bad, return "r -1" )
    DUP 0< IF DROP -1 EXIT THEN    ( bad )
    DUP 9 > IF DROP -1 EXIT THEN   ( bad )
    ( good, add to running result )
    SWAP 10 * +                    ( r*10+n )
    0                              ( good )
;