error  if :
        ( IsUsed[1] && V == 0 ) ||
        ( IsUsed[3] && T == 0 ) ||
        ( IsUsed[0] && IsUsed[2] && p*n==0 && ( p!=0 || n!=0 ) )

undefined if:
        ( IsUsed[0] && IsUsed[2] && p==0 && n==0 )

It's so hard! =)