[plt-scheme] reference to undefined identifier: namespace-variable-binding

I am using signed units in order to parameterize
a heap with different element types.

For testing I want to export the bindings to the top
level like this:

  ; Export functions to the top level
  (namespace-variable-bind/invoke-unit/sig heap^ integer-heap@)

Alas, this results in this error:

  reference to undefined identifier: namespace-variable-binding

Any suggestions?

The program is attached.

Yours,
--
Jens Axel Søgaard
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