f2subf1_fun: Calculate f_2^* - f_1^*
In soonwookwon/rdadapt: What the Package Does (One Line, Title Case)

This function calculates f_2^* - f_1^* that correponds to ω^{-1}(b, Λ_{\mathcal{V}+}(γ_1, C_1),Λ_{\mathcal{V}+}(γ_2, C_2))

1	f2subf1_fun(b, gamma, C, X, mon_ind, swap = FALSE)

`b`	point where the inverse modulus is evaluated at
`gamma`	length two vector of gammas ((γ_1, γ_2)')
`C`	length two vector of Cs ((C_1, C_2)')
`X`	n\timesk design matrix, the kernel is evaluated at each X[i, ]
`mon_ind`	indice of the monotone variables
`swap`	indicator of whether to swap the γ and C.