code: public Iterable<FunctionImport> build(FactoryLookup lookup) {
          List<FunctionImport> builder = new ArrayList<>();
          for (FunctionImportImpl.Builder functionImportBuilder : functionImportBuilders) {
              EntitySet entitySet = lookup.getEntitySet(functionImportBuilder.getEntitySetName());
              Function function = lookup.getFunction(functionImportBuilder.getFunctionName());
              if (entitySet == null && function.isBound()) {
                  throw new IllegalArgumentException("Could not find EntitySet with name: " +
                          functionImportBuilder.getEntitySetName());
              }
              functionImportBuilder.setEntitySet(entitySet);
              functionImportBuilder.setFunction(
                      function);
              builder.add(functionImportBuilder.build());
          }
  
          return Collections.unmodifiableList(builder);
      }
docs: Returns built function imports using passed lookup for searching entity sets and functions for appropriate
  function import.
  @param lookup    lookup to search entity sets and functions for appropriate function import
  @return          built function imports