graphs: how to apply functions for GRAPH to DIGRAPH



I defined a directed graph, exported and imported it in format DIMACS. 
That works fine.
But if I want to check if the graph is planar with function is_planar, I 
get the error "Argument 1 to 'IS_PLANAR' is not a graph: Structure 
[DIGRAPH]. Why is function is_planar not applicable to DIGRAPHs ? How 
can I convert my directed graph to an undirected graph?

Rolf Fankhauser