D, an object of class Digraph (missing documentation), digraph whose sources we are searching for
Outputs:
L, a list, list of all the sources (if there are any)
Description
A source of a Digraph D is a vertex of D that has no parents. That is, v is a source of D if and only if there are only edges pointing from v; none can be pointing into v (there is no edge of the form (v,u)).
i1 : D = digraph({a,b,c,d,e},{{a,b},{b,c},{b,d},{e,b}});