The 'ctrl' modmap is different from the other modmaps as it also applies
the built-in Ctrl modifier to the resulting character, even if it was
first modified by the custom modmap.
For example, this will map Ctrl+в to Ctrl+V (not to v):
<ctrl a="в" b="v"/>
This is intended to add keyboard shortcuts in non-latin layouts.
A caveat is that the latin character appears on the keyboard while Ctrl
is activated.