Say I want to produce a sum of variable values. But I want the running index, say i, to refer to elements of a set of string, like
$$M=\{{Banana, Bread, Computer}\}$$
such that $$i\in M$$ and that the result somehow resembles
$$\sum_{i\in M}{}x_i$$ In my poorer than fair attempt above the upper boundary is missing and I believe the lower boundary is also inadequate to my goal.
I guess my question in short is: How do I define a sum, where the running index refers to strings rather than integers?