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+## unsupported token gluing `foo + bar`
+
+There was a problem in an expression using +, e.g. `foo + bar`.
+This can be due to two causes, check which one applies in your case.
+
+1. You are trying to use + on runtime arguments. Even if you are using
+`foo + bar` in an oper, make sure that the oper isn't called in a
+linearization that takes arguments. Both of the following are illegal:
+
+    lin Test foo bar = foo.s + bar.s          -- explicit + in a lin
+    lin Test foo bar = opWithPlus foo bar     -- the oper uses +
+
+2. One of the arguments in `foo + bar` is a bound variable
+from pattern matching a string, but the cases are non-exhaustive.
+Example:
+    case "test" of {
+      x + "a" => x + "b"   -- no applicable case for "test", so x = ???
+    } ;
+
+You can fix this by adding a catch-all case in the end:
+    { x + "a" => x + "b" ;
+      _       => "default case" } ;
+
+3. If neither applies to your problem, submit a bug report and we
+will update the error message and this documentation.
+
+    https://github.com/GrammaticalFramework/gf-core/issues