Tcl filter readme should include more examples

[giner]

For example, filtering all of Ubuntu 12.04 with not supported kernel (regarding this https://wiki.ubuntu.com/Kernel/LTSEnablementStack#Kernel.2BAC8-Support.A12.04.x_Ubuntu_Kernel_Support):

return [expr [expr [string compare $lsb_rel "12.04"] == 0] && [expr [string first "3.2." $kernel] != 0] && [expr [string first "3.13." $kernel] != 0]]

It looks quite complex, please suggest how to simplify if possible. Can we get rid of expr somwhere? I didn't manage. Links to Tcl filter docs would be also helpful in the readme.

Another example, filtering all of Ubuntu 12.04 wihout linux-generic-lts-trusty installed

return [[expr [string compare $lsb_rel "12.04"] == 0] && [expr [llength [array names installed "linux-generic-lts-trusty"]] == 0]]