Function validate_write 

pub(crate) fn validate_write(
    model: &Model,
    mid: MeasureId,
    coord: &Coordinate,
    value: Option<f64>,
) -> Result<(), ApiError>

S12: a write must target an existing input measure, at a coordinate naming exactly the measure’s categories, each with an item of that category, and carry a finite value (None clears the cell).