Evaluation of the WHERE ... IN ... with multiple columns 

In the case that there is a flattenable subselect with multiple columns, the RowConstructorNode was passed along, and it crashed at the ASTLoader. The fix handles this issue. It builds a branch with AndNodes with on the left side an BinaryComparisonOperatorNode and at the end a Boolean True.  

Extra checks prevent unsupported (and unpredictable) evaluation of cases where a different number of columns are present in the left and right side of the  ... IN ... operator. 
You could have the condition be in an ON clause (`a JOIN b ON (a.x,b.y) IN (SELECT ...)`)