Immutable Pass in Analysis Group

Hi

The LLVM online document says that "There must be exactly one default implementation available at all times for an Analysis Group to be used. Only default implementation can derive from ImmutablePass. "

http://llvm.org/docs/WritingAnLLVMPass.html#implementing-analysis-groups

Is it still true that only the default implementation can derive from ImmutablePass, it does not seem to be the case for TypeBasedAliasAnalysis, which derives from ImmutablePass and is not the default for the AliasAnalysis analysis group in LLVM ?

Thank you,
Trent