Freeciv-manual lists building requirements (maybe also requirements for other objects) so that it outputs nothing when the requirement list is empty. Obsoletion requirement is listed below it so that it says "None" for empty requirement vectors. This gets easily confusing when there's no build requirements but there is obsoletion requirements - so all lines there are about obsoletion requirements. User may take them as build requirements.
Freeciv-manual lists building requirements (maybe also requirements for other objects) so that it outputs nothing when the requirement list is empty. Obsoletion requirement is listed below it so that it says "None" for empty requirement vectors. This gets easily confusing when there's no build requirements but there is obsoletion requirements - so all lines there are about obsoletion requirements. User may take them as build requirements.