Added workaround for a bug introduced by a recent change to Expr class

The removal of baseExpr field from Expr class led to storing all of
baseExpr fields within 'this'. This led to issues especially with
the retrieval of Expr.property (see FIXME in Expr.value())

This commit forces 'this' to take baseExpr.property as its property
in the ARITH constructor case
Working on basic expression handling