public override void ToString(StringBuilder sb)
 {
     sb.Append("ex " + variable + ".");
     sb.Append("(");
     phi.ToString(sb);
     sb.Append(")");
 }
 public override void ToString(StringBuilder sb)
 {
     sb.Append("(");
     left.ToString(sb);
     sb.Append(" | ");
     right.ToString(sb);
     sb.Append(")");
 }
 public override void ToString(StringBuilder sb)
 {
     sb.Append(" \u00AC(");
     phi.ToString(sb);
     sb.Append(")");
 }