Function parse_business_day_convention 

fn parse_business_day_convention(
    named: Option<Named<BusinessDayConvention>>,
) -> BusinessDayConvention

The business-day convention a caller named, or the conventional default.

The default is the core’s own variant, never a string literal on this side. A spelling typed here can disagree with the parser — and did: an earlier draft defaulted to "ModifiedFollowing", which the core spells MODFOLLOWING and rightly refused. Named for the vocabulary it parses, as parse_vol_convention and parse_delta_convention are. The bare convention was the spelling the surface used for five different vocabularies at once.