Encapsulate all the runtime data needed to evaluate a root expression and all its parameters. This class contains the SearchContext that created the root SearchExpression and all the stack frames needed to evaluate all the nested SearchExpression.
| current | Current SearchExpressionContext corresponding to the stack frame being evaluated. |
| frames | The stack of all SearchExpressionContext being evaluated. |
| items | The stack of SearchItems that have been yielded by each execution frame. |
| search | Initial SearchContext contaning the text that was used to parse the initial root SearchExpression. |
| valid | Is the current runtime valid. This means are there any SearchExpressionContext being evaluated. |