S
                            /|\
                           / | \
                          /  |  \
                         /   |   \
                        /    |    \
                       /     |     \
                      /      |      \
                     F       a       S
                    / \             /|\
                   a   F           / | \
                      / \         F  a  S
                     a   F        |     |
                         |        c     b
                         c