Function check_pillars_ascend 

pub fn check_pillars_ascend(
    reference: NaiveDate,
    rungs: &[Rung],
) -> Result<(), CalibrationError>

Every rung’s pillar strictly after the previous one’s, and the first after the reference date.

Errors

CalibrationError::OverlappingPillars naming both rungs by their column labels; CalibrationError::Malformed for a first rung on or before the reference date.