--- tsort_each_child(node) {|child| ...}
#    should be implemented by a extended class.
    TSort ǳĥ륯饹ƤʤФʤʤ᥽åɤǤ

    tsort_each_child is used to iterate for child nodes of node.

#== More Realistic Example
#Very simple `make' like tool can be implemented as follows:

