Failed to verify that effect is installed in current environment

[michaellilltokiwa]

diff --git a/lib/effect.fz b/lib/effect.fz
index d43e5bd5f..33eacaee0 100644
--- a/lib/effect.fz
+++ b/lib/effect.fz
@@ -33,7 +33,7 @@ module:public effect (
   # action to be performed, may include code to run while this effect is installed.
   #
   r effect_mode.val
-  )
+  ) : Effect
 is

   match r
@@ -153,6 +153,23 @@ is
       set res := f()

+  public redef exec(R type, f () -> R) option R =>
+    cf := Effect_Call f
+    abortable cf
+    cf.res
+
+
+public Effect ref is
+  public exec(R type, f () -> R) option R => abstract
+
+
+public with(T type, effects Sequence Effect, code ()->T) option T =>
+  if effects.is_empty
+    code.call
+  else
+    effects.first.exec ()->(with (effects.drop 1) code)
+
+
 # effect mode is an enum that determines how an instance of effect is used
 #
 public effect_mode is

ex is

  greeter (greet String->unit) : simple_effect is

  _ := with [greeter (x -> say "Hello, $x!")] ()->
    greeter.env.greet "World"

leads to:

$MODULE/option.fz:161:27: error 1: Failed to verify that effect 'try (option (option unit))' is installed in current environment.
                | nil => (try (option T)).env.raise (error "error unwrapping option: {option.this}")
--------------------------^^^^^^^^^^^^^^^^^^^
Callchain that lead to this point:

effect environment '--empty--' for call to '(option (option unit)).unwrap' at $MODULE/effect.fz:170:19:
    effects.first.exec ()->(with (effects.drop 1) code)
------------------^^^^
effect environment '--empty--' for call to 'with unit' at /home/sam/playground/test.fz:5:8:
  _ := with [greeter (x -> say "Hello, $x!")] ()->
-------^^^^
effect environment '--empty--' for call to 'ex'
effect environment '--empty--' at program entry

/home/sam/playground/test.fz:6:5: error 2: Failed to verify that effect 'ex.greeter' is installed in current environment.
    greeter.env.greet "World"
----^^^^^^^^^^^
Callchain that lead to this point:

effect environment '--empty--' for call to 'ex.fun.call' at $MODULE/effect.fz:168:10:
    code.call
---------^^^^
effect environment '--empty--' for call to 'with unit' at /home/sam/playground/test.fz:5:8:
  _ := with [greeter (x -> say "Hello, $x!")] ()->
-------^^^^
effect environment '--empty--' for call to 'ex'
effect environment '--empty--' at program entry

2 errors.

but when commenting the error in DfaErrors.java the code executes just fine.