# define a two function argument with default second argument
fn_arg1_less_arg2 <- function(arg1, arg2 = 1) {arg1 - arg2}

# result:  0 1 2 3 4
1:5 %>% fn_two_arg()

# result: -1  0  1  2  3
1:5 %>% fn_two_arg(2)

# result: 4 3 2 1 0
1:5 %>% fn_two_arg(5, .)