collapseos/blk/246

( Place BEGIN, where you want to jump back and AGAIN after
  a relative jump operator. Just like BSET and BWR. )
: BEGIN, PC ;
: BSET PC SWAP ! ;
( same as BSET, but we need to write a placeholder )
: FJR, PC 0 A, ;
: IFZ, JRNZ, FJR, ;
: IFNZ, JRZ, FJR, ;
: IFC, JRNC, FJR, ;
: IFNC, JRC, FJR, ;
: THEN,
    DUP PC          ( l l pc )
    -^ 1-           ( l off )
    ( warning: l is a PC offset, not a mem addr! )
    SWAP ORG @ + BIN( @ - ( off addr )
    C! ;