Text-handling utility.
Flag: Debug mode. (Default 1.)
Interpolated string, emulating Python, e.g.
f'{2} plus {2} is {2+2}'
2 plus 2 is 4
(A B)←2 3
f'{A} plus {B} is {A+B}'
2 plus 3 is 5
An optional left argument is a ref to the context in which expressions are to be evaluated.
The default is the context in which f is called.
With Debug mode on, f signals errors in its argument; with it off, it silently returns its argument.
Errors
123: Nested or unbalanced braces, e.g. '{foo' or '{{foo}}'