Macro require_lt 

macro_rules! require_lt {
    ( $left:expr, $right:expr, $err:expr ) => { ... };
    ( $left:expr, $right:expr ) => { ... };
}

Assert left < right strictly. Anchor-parity sibling of require_gt!. Default error is ProgramError::InvalidArgument because a failed ordering check most often flags a bad user input.