reduceop ()         /* reduce binary op   */
  { TOKEN op, lhs, rhs;
    rhs = popopnd();  /* rhs at top */
    lhs = popopnd();
    op = popop();
    op->operands = lhs; /* first child */
    lhs->link = rhs;    /* next sibling */
    rhs->link = NULL;   /* null terminate */
    pushopnd(op);  } /* subtree now operand */

We use the first child - next sibling form of tree; this represents an arbitrary tree using only two pointers. The tree form of a binary operator and operands is:

               op
              /
             /
   operands /
           /
          /   link
        lhs ----------> rhs

Down arrows are always operands; side arrows are always link. The pretty-printer will print this as:

    (op lhs rhs)