& exch2_txXStackLo,exch2_tyXStackLo,
INTEGER exch2_txXStackLo(W2_maxNbTiles )
exch2_txXStackLo( tId ) = 1 + exch2_tBasex(tId) + fBaseX
tGx = exch2_txXStackLo(tN)
iG = exch2_txXStackLo(tN)+i-1