def fn_x(i, dfs_data):
try:<EOL><INDENT>return R(i, dfs_data)['<STR_LIT:x>']<EOL><DEDENT>except Exception as e:<EOL><INDENT>return dfs_data['<STR_LIT>'].num_nodes()<EOL><DEDENT>
The minimum vertex (DFS-number) in a frond contained in Ri.