Add variable_union_type

Author: blegat

docs/src/types.md

@@ -11,6 +11,7 @@ AbstractVariable
 variable
 name
 name_base_indices
+variable_union_type
 similarvariable
 @similarvariable
 ```

src/variable.jl

@@ -2,6 +2,22 @@ export name, name_base_indices, similarvariable, @similarvariable
 
 Base.copy(x::AbstractVariable) = x
 
+"""
+    variable_union_type(p::AbstractPolynomialLike)
+
+Return the supertype for variables of `p`. If `p` is a variable, it should not
+be the type of `p` but the supertype of all variables that could be created.
+
+### Examples
+
+For `TypedPolynomials`, a variable of name `x` has type `Variable{:x}` so
+`variable_union_type` should return `Variable`.
+For `DynamicPolynomials`, all variables have the same type `PolyVar{C}` where
+`C` is `true` for commutative variables and `false` for non-commutative ones so
+`variable_union_type` should return `PolyVar{C}`.
+"""
+function variable_union_type end
+
 """
     variable(p::AbstractPolynomialLike)