reduceop ()         /* reduce binary op   */
  { TOKEN op, lhs, rhs;
    rhs = popopnd();
    lhs = popopnd();
    op = popop();
    op-> operands = lhs;
    lhs-> link = rhs;
    rhs-> link = NULL;
    pushopnd(op);  }

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)

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