detect whether a sub command's container of a main command is ready for use.
for using this function, the BEGIN and END mixin-macro must be embeed in
main command container.
T is the type in sub-command container, U is the type in main-command container
detect whether a sub command's container of a main command is ready for use. for using this function, the BEGIN and END mixin-macro must be embeed in main command container. T is the type in sub-command container, U is the type in main-command container