Moving graph theory into main part of set.mm (2)
Moving graph theory into main part of set.mm (2)

Moving basic definitions (~df-uhgra, ~df-umgra, ~df-uslgra, ~df-usgra) and corresponding theorems from Mathboxes of Mario and Alexander into a new chapter of the main part of set.mm.

metamath/set.mm,metamath/set.mm,metamath/set.mm