Summation over Finite Subset is Well-Defined

Theorem
Let $\struct{G, +}$ be a commutative monoid.

Let $F \subseteq G$ be a finite subset of $G$.

Then the summation $\ds \sum_{g \mathop \in F} g$ is well-defined.

Proof
To show that summation over $F$ is well-defined it needs to be shown:
 * $(1) \quad \exists$ a finite enumeration of $F$
 * $(2) \quad \forall$ finite enumerations $e$ and $d$ of $F : \ds \sum_{i \mathop = 1}^n e_i = \sum_{i \mathop = 1}^n d_i$

Proof of $(1)$
By definition of finite set:
 * $\exists n \in \N : \exists$ a bijection $e: \closedint 1 n \to F$

Hence $e$ is a finite enumeration of $F$ by definition.

So the summation $\ds \sum_{i \mathop = 1}^n e_i$ exists.

Proof of $(2)$
Let $d: \closedint 1 n \to F$ be any other finite enumeration of $F$.

Consider the composite mapping $e^{-1} \circ d : \closedint 1 n \to \closedint 1 n$ which exists and is a bijection because $e$ and $d$ are bijections.

Let $\operatorname{id}_G: G \to G$ denote the identity mapping on $G$.

We have:

The result follows.