Shape.Base_infoSourcetype t = {summary : Base.string;readme : Base.string Base.option;anons : Anons.t;flags : Flag_info.t Base.list;}find_flag t prefix looks up the flag, if any, to which prefix refers.
It raises if prefix does not begin with - as all flags should.
find_flag does not consider aliases_excluded_from_help, and it assumes that all flags can be passed by prefix. These are limitations in the underlying shape representation.