a!arg is used because I hated the look of a<arg> used in C++. (Using < > to bracket things when < means less than and > means greater than is always trying to short-circuit my brain.)
a<b<arg>>
a(template params)(runtime params) at declaration a!(template params)(runtime params) at invocation with the type deduction and parenthesis omission making often even disappear completely the template syntax.
thank you