Look at the following guix snippet:

(modify-services %desktop-services
  (guix-service-type config => (guix-configuration
       			 (inherit config)
       			 (substitute-urls
       			  (append (list "https://substitutes.nonguix.org")
       				  %default-substitute-urls))
       			 (authorized-keys
       			  (append (list (local-file "/etc/guix/nonguix/signing-key.pub")
       					(local-file "/etc/guix/guix-past/signing-key.pub"))
       				  %default-authorized-guix-keys)))))

The above snippet demonstrates how you can modify services on the fly. In GN, consider a dump:

(define-dump dump-species
  (tables (Species))
  (schema-triples
   (gn:name rdfs:range rdfs:Literal)
   (gn:displayName rdfs:range rdfs:Literal)
   (gn:binomialName rdfs:range rdfs:Literal)
   (gn:family rdfs:range rdfs:Literal))
  (triples (string->identifier "species" (field Species FullName))
    (set rdf:type 'gn:species)
    (set gn:name (field Species SpeciesName))
    (set gn:displayName (field Species MenuName))
    (set gn:binomialName (field Species FullName))
    (set gn:family (field Species Family))
    (set gn:organism (ontology 'ncbiTaxon: (field Species TaxonomyId)))))

We could have all the dumps in one file, say:

%genenetwork-tables-dump

which have the default dumps that were defined earlier. Should someone want to change something, one possible interface would be:

(modify-dump %genenetwork-tables-dump
	     (dump-species schema-triples =>
			   (schema-triples
			    (inherit schema-triples)
			    (replace (gn:name rdfs:range rdfs:Literal)
			      (gn:name rdfs:range gn:something-