#' Peform SUL calculation
#'
#' @param x a data frame that contains var, genderVar and bmi columns 
#' @param var a variable measured in SUV that should be converted to SUL
#' @param sulVar target variable to store converted values
#' @param genderVar a variable denoting gender (1=female, 2=male)
#' @param bmiVar a variable with body mass index values
#'
#' @return data frame x with added sulVar column
#'
#' @export

convert.to.sul<-function(x,var,sulVar,genderVar='gender',bmiVar='bmi'){
   x[,sulVar]=x[,var]
   male=x[,genderVar]==2
   female=x[,genderVar]==1
   x[male,sulVar]=x[male,sulVar]*(1.1-0.0128*x[male,bmiVar])
   x[female,sulVar]=x[female,sulVar]*(1.07-0.0148*x[female,bmiVar])
   x
}