Return a StateMachineInstance if the Symbol represented
by the symbol index is valid as the initial symbol of
The returned StateMachineInstance will have its statepointer
updated to show receipt of the specified symbol.
This method should not be used outside
of the package as it does no alphabet checks at all.
It should be package-private except I cannot define
an interface with such methods.
symIdx - alphabet index value for specified symbol.
an instance of StateMachineInstance if symbol
is valid otherwise null.