Fix mutation of power of monomial (#142)

Author: blegat

src/operators.jl

@@ -4,7 +4,7 @@
 # Defining the specific methods solve this problem and also make
 # them a lot faster
 Base.:(^)(x::Variable{V,M}, i::Int) where {V,M} = Monomial{V,M}([x], [i])
-Base.:(^)(x::Monomial{<:Commutative}, i::Int) = Monomial(x.vars, i * x.z)
+Base.:(^)(x::Monomial{<:Commutative}, i::Int) = Monomial(copy(x.vars), i * x.z)
 
 myminivect(x::T, y::T) where {T} = [x, y]
 function myminivect(x::S, y::T) where {S,T}

test/runtests.jl

@@ -12,6 +12,14 @@ using LinearAlgebra
     @test p(x0 => y0, x1 => y1) == y1 * y0 * y1
 end
 
+# https://github.com/JuliaAlgebra/DynamicPolynomials.jl/issues/141
+@testset "Issue #141" begin
+    @polyvar x
+    m = x^2
+    q = m^2
+    @test variables(q) !== variables(m)
+end
+
 @testset "Issue #79, Issue #80 and Issue #92" begin
     @polyvar x[1:2]
     p1 = x[1] * 0.0 + x[2] * 0