In s() and t2() terms, arguments other than predictors are not al…

…lowed,

so throw an appropriate error message.

This is a follow-up for issue stan-dev#156 which was only concerned with documentation.
However, this still does not address stan-dev#238 yet (but at least we now throw an
appropriate error message).

R/formula.R

@@ -108,6 +108,10 @@ parse_additive_terms <- function(terms) {
   smooth <- unname(unlist(sapply(smooth_terms, function(et) {
     grep(make_function_regexp(et), terms, value = TRUE)
   })))
+  if (any(grepl("\\(.+,.+=.+\\)", smooth))) {
+    stop("In s() and t2() terms, arguments other than predictors are not ",
+         "allowed.")
+  }
   return(smooth)
 }