ref: 402741bc017cfa9602e7d8ea8645696ace54f4cd
dir: /sys/src/libcontrol/group.h/
typedef struct Group Group; struct Group { Control; int lastbut; int border; int mansize; /* size was set manually */ int separation; int selected; int lastkid; CImage *bordercolor; CImage *image; int nkids; Control **kids; /* mallocated */ Rectangle *separators; /* mallocated */ int nseparators; };