How to force a whitespace?

[Laxystem]

Say I have this rule:

WHITESPACE = _{ PATTERN_WHITESPACE }
DECLARATION = { KEYWORD ~ CONTENTS }

How do I force a WHITESPACE between them? Simply doing ~ WHITESPACE ~ doesn't seem to work.

[Laxystem]

Managed to solve it by making DELCARATION compound-atomic ($) and CONTENTS non-atomic (!).