Are sub-strings strings?

[PhilOfPerth]

I often need to swap two sections of a string, as in the snippet below.
Wiki says "The SWAP statement is used to exchange two variable or array element values".
I find I can only do this by using a temporary variable, so I need 3 lines of code instead of 1.

Is a part of a string a variable? 
If not, maybe the Wiki could clarify this, to save L-Platers like me a lot of time experimenting?

(This is a very simple example, but others can be more significant).
T$ = "ABCDEFG"
Print T$
Swap mid$(T$,2,1) ,mid$(T$,5,1)                   <--   this line is not allowed
Print T$

Tmp$ = Mid$(T$, 2, 1)
Mid$(T$, 2, 1) = Mid$(T$, 5, 1)
Mid$(T$, 5, 1) = Tmp$
Print T$