All of the examples that I can find have absolute paths for the left
hand side of directives. Does this have to be the case ?

can I easily wrire a directive that, for example doesn't back up
anything called cache in a directory called .mozilla

<< .mozilla >>
    +null: cache


The problem here is that cache can be one or more levels down inside a
.mozilla directory and home directories aren't  always at the top level

for home directories at the top level, I assume I could write

<< / >>
    +null .mozilla/*/*/cache
    +null .mozilla/*/*/*/cache

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