Add a comment explaining default inhabitants

As per https://github.com/mit-plv/fiat-crypto/pull/332/files#r176422391

1 files changed, 5 insertions, 0 deletions

diff --git a/src/Experiments/SimplyTypedArithmetic.v b/src/Experiments/SimplyTypedArithmetic.v
index 5805741d6..f3c734998 100644
--- a/src/Experiments/SimplyTypedArithmetic.v
+++ b/src/Experiments/SimplyTypedArithmetic.v
@@ -4069,6 +4069,11 @@ Module Compilers.
   End ZRange.
 
   Module DefaultValue.
+    (** This module provides "default" inhabitants for the
+        interpretation of PHOAS types and for the PHOAS [expr] type.
+        These values are used for things like [nth_default] and in
+        other places where we need to provide a dummy value in cases
+        that will never actually be reached in correctly used code. *)
     Module type.
       Module primitive.
         Definition default {t : type.primitive} : type.interp t