Question re symbols, symtabs and blocks

Hi,

in GDB, are there any constraints for blocks belonging to
to particular symtab? Like: must each blocks' start and end address
be within its superblocks' start and end address? 

Also, must blocks of different symtabs be always disjunct 
(meaning, their start-end ranges must not overlap)? 

Here by "must" I mean "the code expects that and if
it does not hold, behavior is undefined".

Thanks! Jan