Function read 

pub(crate) fn read(
    swap: El<'_, '_>,
    ctx: &Context<'_>,
) -> Result<(AnyInstrument, [usize; 2]), FpmlError>

Reads a swap element into a swap, us being the reporting party’s id, with the zero-based leg index each stream was read into: a swap orders its legs by role (fixed then floating, or paid then received), not by the document’s stream order.