Strings
    are enclosed by double or two single quotes:
        "foo"
        ''foo''
    Interpolation has only one form:
        nix-repl> foo = "strval" // special repl-only syntax assignment
        nix-repl> "${foo}"
            "strval"

        nix-repl> "${2+3}"
            error: cannot coerce an integer to a string, at (string):1:2
            // need to call integer -> string explicit conversions

    Escaping ${...} within double quoted strings is done with the backslash.

        nix-repl> "\${foo}"
            "${foo}"

    Escaping ${...} within two single quotes is done with '': 
        nix-repl> ''test ''${foo} test''
            "test ${foo} test"