code: function(pos, expr, succeeded, bindings) {
      if (expr instanceof pexprs.Apply) {
        var app = this.currentApplication();
        var actuals = app ? app.args : [];
        expr = expr.substituteParams(actuals);
      }
      return this.getMemoizedTraceEntry(pos, expr) ||
             new Trace(this.input, pos, this.inputStream.pos, expr, succeeded, bindings, this.trace);
    }
docs: Returns a new trace entry, with the currently active trace array as its children.