[R] generating sequences with gaps

[etienne]

I need sequences that have gaps in them, such as the
following:

4 5 6 | 12 13 14 | 20 21 22

a simple question, I've been scratching my head for a
R function that will do this

The : and seq do not allow this, and the c() can be
used although not in an automatic way.  I'm sure there
is a way to do it without using a for() construct.

Thank you