Fix substitute promotion

src/substitution.jl

@@ -6,19 +6,37 @@ MP.substitute(st::MP.AbstractSubstitutionType, v::Variable, s::Substitution) = v
 # subs(x, x=>y, y=>1) should be y, not 1 so as soon as we see the right variable, we stop, in subs(x, x=>y, x=>1), "x=>1" is ignored.
 MP.substitute(st::MP.AbstractSubstitutionType, v::Variable{Name}, s::Substitution{Name}, ::MP.AbstractSubstitution...) where {Name} = s.second
 
-_remove_variable(t::Tuple{}, ::Type) = t
+_remove_variable(::Tuple{}, ::Type) = nothing
 function _remove_variable(t::Tuple{V,Vararg{Variable,N}}, ::Type{V}) where {V,N}
     Base.tail(t)
 end
 function _remove_variable(t::Tuple{V,Vararg{Variable,N}}, ::Type{W}) where {V,W,N}
-    tuple(first(t), _remove_variable(Base.tail(t), W)...)
+    tail = _remove_variable(Base.tail(t), W)
+    if isnothing(tail)
+        return
+    else
+        return tuple(first(t), tail...)
+    end
 end
 
 _mult_monomial_type(::Type{U}, ::Tuple{}) where {U} = U
 _mult_monomial_type(::Type{U}, V::Tuple) where {U} = MA.promote_operation(*, U, Monomial{V,length(V)})
 
-function MA.promote_operation(::typeof(MP.substitute), ::Type{MP.Subs}, ::Type{Monomial{V,N}}, ::Type{Pair{Variable{Name},T}}) where {V,N,Name,T}
+function _promote_subs(::Type{Monomial{V,N}}, ::Type{Pair{Variable{Name},T}}) where {V,N,Name,T}
     U = MA.promote_operation(*, T, T)
     VV = _remove_variable(V, Variable{Name})
-    return _mult_monomial_type(U, VV)
+    if isnothing(VV)
+        # Variable not present
+        return Monomial{V,N}
+    else
+        return _mult_monomial_type(U, VV)
+    end
+end
+
+function MA.promote_operation(::typeof(MP.substitute), ::Type{MP.Subs}, ::Type{Monomial{V,N}}, ::Type{Pair{Variable{Name},T}}) where {V,N,Name,T}
+    return _promote_subs(Monomial{V,N}, Pair{Variable{Name},T})
+end
+
+function MA.promote_operation(::typeof(MP.substitute), ::Type{MP.Eval}, ::Type{Monomial{V,N}}, ::Type{Pair{Variable{Name},T}}) where {V,N,Name,T}
+    return _promote_subs(Monomial{V,N}, Pair{Variable{Name},T})
 end