main and subtags?

edit

Is it possible for automatic sub-tagging, perhaps by editing the part of the hard-code myself?

physics

1. magnetism
2. gravitity
3. electicity

math

1. calculus
2. geometry
3. algebra

What I want to accomplish. For every "manual tag" automatically retag with maintag.

e.g. "calculus" => "calculus, math"

e.g. "gravity" => "gravity, physics"

e.g. "gravity, physics" => "gravity, physics"