diff --git a/src/core/PocomathInstance.mjs b/src/core/PocomathInstance.mjs
index 5640e20..307f832 100644
--- a/src/core/PocomathInstance.mjs
+++ b/src/core/PocomathInstance.mjs
@@ -33,7 +33,14 @@ export default class PocomathInstance {
     *    by the signature and returning the value. Otherwise, it should be
     *    a function taking an object with the dependency lists as keys and the
     *    requested functions as values, to a function taking the arguments
-    *    specified by the signature and returning the value
+    *    specified by the signature and returning the value.
+    *
+    *    Note that the "operation" named `Types` is special: it gives
+    *    types that must be installed in the instance. In this case, the keys
+    *    are type names, and the values are objects with a property 'test'
+    *    giving the predicate for the type, and properties for each type that can
+    *    be converted **to** this type, giving the corresponding conversion
+    *    function.
     */
    install(ops) {
       for (const key in ops) this._installOp(key, ops[key])