Need warning when users enter circular expressions

[ACowan0105]

e.g. Kr = Kd*Kr should show warning to users

[jcschaff]

@danv61 has this been fixed alread?

[danv61]

@jcschaff It's not fixed in kinetic laws. Kr=2+Kr is accepted, also infinite circular reference between Kf and Kr. Anyway, testing some very complicated indirect expressions may become difficult to catch