Syntax: group which can be contained or uncontained

I'm trying to write my own syntax file for a language where there are a lot of groups of the form

:syn match langSomeGrp /^\s*KeyWord.*/

The problem is that there are also end-of-line comments
:syn match langComment /--.*/

which can either stand on a line of its own OR be contained in a group like the above. How do I express that? Is it OK to have a contains=grpB in the declaration of a match grpA without a containedin=grpA in the declaration of match grpB?

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