Question: Search for an explanation about CycleBasis

Could someone please explain to me why the procedure GraphTheory:-CycleBasis requires that the vertices of a graph be integers for it to work correctly ?
(while the definition of a graph supports other types of vertex names)


restart:
with(GraphTheory):

#------------------------------------------------------- test 1

g1 := Graph({{"a", 1}, {1, 3}, {"a", 3}});
DrawGraph(g1);
CycleBasis(g1);

Error, (in GraphTheory:-CycleBasis) cannot determine if this expression is true or false: "a" < 1

#------------------------------------------------------- test 2

g2 := Graph({{a, 1}, {1, 3}, {a, 3}});
CycleBasis(g2);

Error, (in GraphTheory:-CycleBasis) cannot determine if this expression is true or false: a < 1

#------------------------------------------------------- test 3

g3 := Graph({{4, 1}, {1, 3}, {4, 3}});
CycleBasis(g3);

[[1, 3, 4]]

 
Knowing this limitation I usually rename to integers the vertices of the graphs, next do some operations, and next go back to the original names.


Thanks for any explanation.