Using an "update" class in "main"

[Mr. Print]

I would like to have the performance of an action depend on whether
any of the cfengine configuration files (eg. cf.classes) was updated
from the policy server.

In update.conf I have added "define=CfFilesChanged" to the section
which handles /var/cfengine/inputs/*.

Although this class seems to be defined during the "updates" context,
it is not defined in the "main" context.

What am I doing wrong?