After this commit, a value of type 'expr' cannot be a reference to nullptr.
This commit also fixes several bugs due to the use of 'null' expressions.
TODO: do the same for kernel objects, sexprs, etc.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
We flat applications. So, (g a b) is actually ((g a) b).
So, we must be able to unify (?f ?x) with (g a b).
Solution:
?g <- (g a)
?x <- b
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
