simplify fails on certain trig identities and adding a new rule does not work

[danielpporras]

I would like to simplify 2 - 2cos(x)^2 to 2sin(x)^2. However, simplify does not work because it does not apply the identity. I attempted to write a new rule in several ways such as @acrule 1 + -1cos(~x)^2 => sin(~x)^2 but that has not yet worked. I, of course, want the rule work for any constant a such that a - a*cos(x)^2 goes to a*sin(x)^2.

julia> using SymbolicUtils
julia> @syms x
(x,)
julia> simplify(2 - 2cos(x)^2)
2 - 2(cos(x)^2)
julia> simplify(2 - 2cos(x)^2, @acrule 1 + -1cos(~x)^2 => sin(~x)^2)
2 - 2(cos(x)^2)