Minimum Reproducible Example: Stan Include File

Error message

Compiling Stan program...
Syntax error in '/var/folders/mw/nw_yn9hs0wd6flz32fwr81sh0000gn/T/RtmphVRHT8/model-9541147ec224.stan', line 2, column 0, include error:
   -------------------------------------------------
     1:  functions {
     2:  #include multiply.stan
         ^
     3:  }
     4:  data {
   -------------------------------------------------

Could not find include file multiply.stan in specified include paths.

make: *** [/var/folders/mw/nw_yn9hs0wd6flz32fwr81sh0000gn/T/RtmphVRHT8/model-9541147ec224.hpp] Error 1

Error: An error occured during compilation! See the message above for more information.

example.R

library(cmdstanr)

cmdstan_model("./model.stan")
cmdstan_model("./model.stan", include_paths = ".")
cmdstan_model("./model.stan", include_paths = "~/stanmre")
cmdstan_model("./model.stan", include_paths = "~/stanmre/")
cmdstan_model("./model.stan", include_paths = "/Users/herb/stanmre")

cmdstanr::cmdstan_version()
sessionInfo()

model.stan

functions {
#include multiply.stan
}
data {
  int<lower=0> N;
  vector[N] y;
}
parameters {
  real mu;
  real<lower=0> sigma;
}
model {
  y ~ normal(mu, sigma);
}

multiply.stan

real multiply(real x, real y) {
  return x * y;
}