% example44.sil %
X:=0;Y:=?;R:=?;B:=?;
while ((Y<-1) | (Y>1))  do
  Y:=?
od;
while (B<>R) do
  while ((R<-1) | (R>1))  do
    R:=?
  od;
  if (X=Y) then
    Y:=X+R
  else
    X:=Y+R
  fi 
od;;