confused about setting parameters/variables in dhcpd.conf and "set" keyword

[btb at bitrate.net]

i've seen the "set" keyword used in various discussions on the ml and in a handful of config examples i've found here and there.  this does appear to work - however, i can't seem to find a formal reference to this in any of man 5 dhcpd.conf, man 5 dhcp-eval, or man 5 dhcp-options.  am i missing it somewhere?

i did find, in man 5 dhcpd.conf, the section "SETTING PARAMETER VALUES USING EXPRESSIONS" - but that seems to strictly apply to setting values of specific predefined "dhcp server parameters"?  testing just "my-parameter = expression ;" appears to indeed result in no value being set for my-parameter, where my-parameter is some arbitrary "custom" string.

finally, i've noticed that if i set some custom parameter with "set foo = "bar";, this assignment appears to persist, even if i remove the line from the config and restart the dhcpd service - for example, if i log() the variable, i still see its value, even if the set line is removed from the config.  where is this assignment stored?  how can i remove/unset a variable that is no longer desired?

thanks
-ben