Cannot groupBy multiple unnamed expression columns

[Jolanrensen]

Cannot groupBy multiple unnamed expression columns:

dataFrameOf("a", "b")(1, 2)
    .groupBy { expr { "a"<Int>() + 1 } and expr { "b"<Int>() - 1 } }

results in an exception

Can not insert more than one column into the path []
java.lang.IllegalStateException: Can not insert more than one column into the path []

Seems that groupBy doesn't find unique names for unnamed columns, like DataFrameImpl.init does.