static void tok_unpair(L<S> tok) {
  int i;
  while ((i = jfind(tok, "<id> <id>, <id> <id> = unpair")) >= 0) {
    int idx = indexOf(tok, "unpair", i);
    int j = findEndOfStatement(tok, idx);
    S type1 = tok.get(i), var1 = tok.get(i+2);
    S type2 = tok.get(i+6), var2 = tok.get(i+8);
    S v = makeVar();
    tok.set(i+4, ";");
    tok.set(idx-2, ";");
    tok.set(i+2, "Pair<" + type1 + "," + type2 + "> " + v + "=");
    tok.set(j-1, "; " + var1 + " = " + v + ".a; " + var2 + " = " + v + ".b;");
    reTok(tok, i, j);
  }
  
  while ((i = jfind(tok, "<id> <id>, <id><<id>,<id>> <id> = unpair")) >= 0) {
    int idx = indexOf(tok, "unpair", i);
    int j = findEndOfStatement(tok, idx);
    S type1 = tok.get(i), var1 = tok.get(i+2);
    S type2 = joinSubList(tok, i+5, idx-5), var2 = tok.get(idx-4);
    S v = makeVar();
    tok.set(i+4, ";");
    tok.set(idx-2, ";");
    tok.set(idx-1, "");
    tok.set(idx, "Pair<" + type1 + "," + type2 + "> " + v + "=");
    tok.set(j-1, "; " + var1 + " = " + v + ".a; " + var2 + " = " + v + ".b;");
    reTok(tok, i, j);
  }
}