"{$user}"                                  # a parameter
"{$count * 2}"                             # any C++ expression
"{$size;std::fixed;std::setprecision(1)}"  # with formats
"{$n;std::setw(3);fill('0')}"              # manipulator + member call
"{$ std::string{ \"br{}ace\" } }"          # braces nest

• The expression is any C++ expression valid in the generated function — usually a parameter. Braces inside it nest; a ; or an unbalanced } can be written \; / \}.
• Each format is a statement applied to the stream before the value:
• width(…), precision(…), fill(…), setf(…), unsetf(…), flags(…), imbue(…), or anything starting with ., is a member call — stream.precision(1);
• anything else is a manipulator — stream << std::setw(3);
• Afterwards the stream's whole format is restored, so a format never leaks into the rest of the text.

Macros work in double-quoted and bar lines, not in single-quoted ones. Manipulators with arguments need $INCLUDE <iomanip>.

app.msg:2:4: error: unterminated macro, missing '}'
app.msg:2:4: error: empty macro expression