add to-do item

TODO

@@ -2,6 +2,12 @@ Roughly in order of importance.                                     -*-org-*-
 
 * Fix lattice.neighbors() when lattice has lower dimensionality than the space.
 
+* Improve error messages by checking for ABCs
+  Add a classmethod "ensure_isinstance" that takes an object at makes sure that
+  it has the appropriate type.  Otherwise, a non-cryptic error is raised.  The
+  error message can depend on the type of the argument.  For example, it could
+  inform that a builder needs to be finalized.
+
 * Document the order of sites/orbitals in finalized builders
 
 * Add calculation of current density