String-instrument string numbers use circled numerals, distinct from finger numbers

For string instruments, the string being played is notated with a circle around the numeral, visually distinct from a bare numeral indicating left-hand finger — conflating the two is a real source of performer confusion the convention specifically guards against.

A string number is authored/rendered as a bare numeral identical in appearance to a finger number, with no circle, making it ambiguous whether it means finger or string.

Gould explicitly distinguishes fingering numerals (bare) from string numbers (circled) for string instruments; Read similar.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.