In Arrears 1

Key In Arrears 1 in
IRS refers to an optional array of integers that determine if the Libor rate is set at the end of its respective accrual period.
0 <-> FALSE and 1 <-> TRUE
If omitted, it is assumed to contain 0, which means the Libor is set in advance.
Note, the 1 case requires a vol input in the pricing function in order to compute the corresponding convexity adjustment.