Function lower_rows 

pub fn lower_rows(
    conjuncts: &[Expr],
    arrays: &[ArrayState],
    sizes: &[usize],
    states: &[Option<BigUint>],
) -> Result<Lowered, ExpandError>

Binds the rows of every array of arrays for the second sizing stage (module docs of ArrayNode::RowScope): sizes are the outer sizes the first stage chose. Every array of arrays gets one row array per row (the row_base ids), an array of arrays becomes inactive with that many placeholder elements, and the conjuncts name rows directly.