Currently, the left hand side of a comparison is always a column.
However, it should be possible to create comparison where no explicit column is involved (see
TORQUE-185 for an example)
All values which are currently possoble for the right hand side of the comparsion (ex field "value") should also be possible for the left hand side.
This leads to the change in semantics that Strings als lhs are not interpreted as columns any more but as values.