Proof of proveit.logic.sets.functions.images.function_image_is_within_codomain theorem

import proveit
theory = proveit.Theory() # the theorem's theory

%proving function_image_is_within_codomain

With these allowed/disallowed theorem/theory presumptions (e.g., to avoid circular dependencies), we begin our proof of
function_image_is_within_codomain:
(see dependencies)