Document number of inputs for alternatives.

diff --git a/theme.rst b/theme.rst
index 3fa5873587a0a665c8ea0061283983eb452fa3f5..d43687fc76df8e44add1021a200784068a090665 100644 (file)
--- a/theme.rst
+++ b/theme.rst
@@ -184,7 +184,10 @@ directly by the GPU) that instead. The scaling is set up like this::
   -- Or by numerical index:
   input0_scaled:choose(1)  -- Chooses ResizeEffect
 
-Note that add_effect returns its input for convenience.
+Note that add_effect returns its input for convenience. All alternatives must
+have the same amount of inputs, with an exception for IdentityEffect, which can
+coexist with an effect requiring any amount of inputs (if selected, the IdentityEffect
+just passes its first input unchanged).
 
 Actually, add_optional_effect() is just a wrapper around add_effect() with
 IdentityEffect as the other alternative, and disable() is a convenience version of