test_that("rxFix", {

  One.comp.transit.allo <- function() {
    ini({
      # Where initial conditions/variables are specified
      lktr <- log(1.15)  #log k transit (/h)
      lcl  <- log(0.15)  #log Cl (L/hr)
      lv   <- log(7)     #log V (L)
      ALLC <- fix(0.75)  #allometric exponent cl
      ALLV <- fix(1.00)  #allometric exponent v
      prop.err <- 0.15   #proportional error (SD/mean)
      add.err <- 0.6     #additive error (mg/L)
      eta.ktr ~ 0.5
      eta.cl ~ 0.1
      eta.v ~ 0.1
    })
    model({
      #Allometric scaling on weight
      cl <- exp(lcl + eta.cl + ALLC * logWT70)
      v  <- exp(lv + eta.v + ALLV * logWT70)
      ktr <- exp(lktr + eta.ktr)
      # RxODE-style differential equations are supported
      d/dt(depot)   = -ktr * depot
      d/dt(central) =  ktr * trans - (cl/v) * central
      d/dt(trans)   =  ktr * depot - ktr * trans
      ## Concentration is calculated
      cp = central/v
      # And is assumed to follow proportional and additive error
      cp ~ prop(prop.err) + add(add.err)
    })
  }

  tmp <- rxFixPop(One.comp.transit.allo)

  expect_equal(tmp$theta,
               c(lktr = 0.139761942375159, lcl = -1.89711998488588, lv = 1.94591014905531,
                 prop.err = 0.15, add.err = 0.6))

  tmp2 <- rxFixPop(tmp)

  expect_equal(tmp2$theta,
               c(lktr = 0.139761942375159, lcl = -1.89711998488588, lv = 1.94591014905531,
                 prop.err = 0.15, add.err = 0.6))

  tmp3 <- rxFixPop(tmp2, returnNull = TRUE)

  expect_true(is.null(tmp3))

  nlmixr_threecmt_mm_no_add_wtcl_pdtg_kout_delay2 <- function() {
    ini({
      tf_sc <- log(999)
      tf_infilt <- log(999)
      tka_sc <- log(999)
      tka_infilt <- log(999)
      tcl_low <- log(999)
      tcl_high <- log(999)
      tcl_c50 <- log(3000)
      e_wt_cl <- fixed(999)
      tv <- log(999)
      tq1 <- log(999)
      tvp1 <- log(10)
      tq2 <- log(999)
      tvp2 <- log(20)
      eta_cl~999
      eta_v~999
      prop_err <- 999
      tg_bl <- log(999)
      eta_tg_bl~999
      tg_kel <- log(999)
      tg_ec50 <- log(5000)
      tg_emax_kel <- log(2)
      ktr_tg <- log(999)
      prop_err_tg <- 999
    })
    model({
      # PK setup
      f_sc <- exp(tf_sc)
      f_infilt <- exp(tf_infilt)
      ka_sc <- exp(tka_sc)
      ka_infilt <- exp(tka_infilt)
      cl_low <- exp(tcl_low + eta_cl)*(WEIGHT_BL/85)^e_wt_cl
      cl_high <- exp(tcl_high + eta_cl)*(WEIGHT_BL/85)^e_wt_cl
      cl_c50 <- exp(tcl_c50)
      v <- exp(tv + eta_v)
      q1 <- exp(tq1)
      vp1 <- exp(tvp1)
      q2 <- exp(tq2)
      vp2 <- exp(tvp2)
      # PK micro-parameters
      ke_low <- cl_low/v
      ke_high <- cl_high/v
      kc_p1 <- q1/v
      kp1_c <- q1/vp1
      kc_p2 <- q2/v
      kp2_c <- q2/vp2
      # TG setup
      tgbl <- exp(tg_bl + eta_tg_bl)
      kin_tg <- tgbl*exp(tg_kel)
      ktr_TG <- exp(ktr_tg)
      TG(0) <- tgbl
      # differential equations
      cp <- CENTRAL/v*1e3 # 1e3 is for unit conversion
      ke <- ke_low + (ke_high - ke_low)*cp/(cp + cl_c50)
      kout_tg <- exp(tg_kel) + exp(tg_emax_kel)*TG_TR/(TG_TR + exp(tg_ec50))
      d/dt(IVINFILT) =             - ka_infilt * IVINFILT
      d/dt(SC)       = -ka_sc * SC
      d/dt(CENTRAL)  =  ka_sc * SC + ka_infilt * IVINFILT - ke*CENTRAL - kc_p1*CENTRAL + kp1_c*P1 - kc_p2*CENTRAL + kp2_c*P2
      d/dt(P1) =                                                         kc_p1*CENTRAL - kp1_c*P1
      d/dt(P2) =                                                                                    kc_p2*CENTRAL - kp2_c*P2
      f(SC) <- f_sc
      f(IVINFILT) <- f_infilt
      # TG transit model
      d/dt(TG_TR) = ktr_tg*cp - ktr_tg*TG_TR
      d/dt(TG) = kin_tg - kout_tg*TG
      # Residual error models
      cp ~ prop(prop_err)
      TG ~ prop(prop_err_tg)
    })
  }

  tmp <- rxFixPop(nlmixr_threecmt_mm_no_add_wtcl_pdtg_kout_delay2)

  expect_equal(tmp$theta,
               c(tf_sc = 6.90675477864855, tf_infilt = 6.90675477864855, tka_sc = 6.90675477864855,
                 tka_infilt = 6.90675477864855, tcl_low = 6.90675477864855, tcl_high = 6.90675477864855,
                 tcl_c50 = 8.00636756765025, tv = 6.90675477864855, tq1 = 6.90675477864855,
                 tvp1 = 2.30258509299405, tq2 = 6.90675477864855, tvp2 = 2.99573227355399,
                 prop_err = 999, tg_bl = 6.90675477864855, tg_kel = 6.90675477864855,
                 tg_ec50 = 8.51719319141624, tg_emax_kel = 0.693147180559945,
                 ktr_tg = 6.90675477864855, prop_err_tg = 999))


})