Represents an expression bound to a returned Field with the given
name. FieldExpressions are added to a
, to request an
expression computed and returned as the named Field value. For example,
.setExpression("snippet(\"good story\", content)")
binds a snippet expression to a Field named "snippet", which will
be returned in each search result. In this case the returned "snippet"
Field will contain a HTML value containing text snippets of the
"content" field matching the query "good story".