feat(builtin/kernel): add nonempty_fun theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(builtin/kernel): add nonempty_range theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/simplifier): missing condition in implication simplification

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(examples/lean): "dependent" if-then-else example

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
doc(examples/lean): "dependent" if-then-else example