Check if State applied when typing formualas

I am using Yanfly's skill core: http://yanfly.moe/2015/10/13/yep-8-skill-core/ and I wanted to make a skill that costs 18 MP normally, but only costs 3 MP is the state "Blue aura" is applied.

I tried:

if a.state?(15)

cost = 3

 but that didn't work. What would I type instead to find if the user has state 15 applied.

a.isStateAffected(15)

Thank you so much!

You saved me about so many hours of trial and error.