Removed rowwise operations, they are super slow

    return(res)

}

.merge_betas <- function(X1, X2) {

    terms_join <- setdiff(intersect(colnames(X1), colnames(X2)), 'beta')

    suppressMessages({

        dplyr::full_join(X1, X2, by = terms_join) %>% 

            dplyr::rowwise() %>% 

            dplyr::mutate(beta = sum(beta.x, beta.y, na.rm = TRUE)) %>% 

            dplyr::ungroup() %>% 

            dplyr::select(-beta.x, -beta.y)  

        res <- dplyr::full_join(X1, X2, by = terms_join) 

        res$beta <- replace_na(res$beta.x, 0) + replace_na(res$beta.y, 0)

        res$beta.x <- NULL

        res$beta.y <- NULL        

    })

    return(res)

}

.merge_sigmas <- function(X1, X2) {

    terms_join <- setdiff(intersect(colnames(X1), colnames(X2)), 'sigma')

    suppressMessages({

    dplyr::full_join(X1, X2, by = terms_join) %>% 

        dplyr::rowwise() %>% 

        dplyr::mutate(sigma = sqrt(sum(sigma.x ^ 2, sigma.y ^ 2, na.rm = TRUE))) %>% 

        dplyr::ungroup() %>% 

        dplyr::select(-sigma.x, -sigma.y)

        res <- dplyr::full_join(X1, X2, by = terms_join) 

        res$sigma <- sqrt(replace_na(res$sigma.y, 0) ^ 2 + replace_na(res$sigma.y, 0) ^ 2)

        res$sigma.x <- NULL

        res$sigma.y <- NULL        

    })

    return(res)

}