: _pd ( a -- n f, parse decimal )
    C@+ OVER C@ 0 ( a len firstchar startat )
( if we have '-', we only advance. more processing later. )
    SWAP '-' = IF 1+ THEN ( a len startat )
( if we can do the whole string, success. if _pdacc returns
  false before, failure. )
    0 ROT> ( len ) ( startat ) DO ( a r )
        OVER I + C@ ( a r c ) _pdacc ( a r f )
        NOT IF DROP 1- 0 UNLOOP EXIT THEN LOOP ( a r )
( if we had '-', we need to invert result. )
    SWAP C@ '-' = IF 0 -^ THEN 1 ( r 1 ) ;