code: function(pos, expr) {
      var posInfo = this.memoTable[pos];
      if (posInfo && expr.ruleName) {
        var memoRec = posInfo.memo[expr.toMemoKey()];
        if (memoRec && memoRec.traceEntry) {
          var entry = memoRec.traceEntry.cloneWithExpr(expr);
          entry.isMemoized = true;
          return entry;
        }
      }
      return null;
    }
docs: Returns the memoized trace entry for `expr` at `pos`, if one exists, `null` otherwise.