old_contents: #! /usr/bin/env python3
  
  import os
  import sys
  
  import runtime
  import module
  
  def main():
      r = runtime.Runtime()
      peru_file_name = os.getenv("PERU_FILE_NAME") or "peru"
      m = module.parse(r, peru_file_name)
      if len(sys.argv) > 1:
          target = sys.argv[1].split('.')
      else:
          target = []
      m.build(r, target)
  
  if __name__ == '__main__':
      main()
  
new_contents: #! /usr/bin/env python3
  
  import os
  import sys
  
  import runtime
  import module
  
  def main():
      peru_file_name = os.getenv("PERU_FILE_NAME") or "peru"
      if not os.path.isfile(peru_file_name):
          print("no peru file found")
          sys.exit(1)
      r = runtime.Runtime()
      m = module.parse(r, peru_file_name)
      if len(sys.argv) > 1:
          target = sys.argv[1].split('.')
      else:
          target = []
      m.build(r, target)
  
  if __name__ == '__main__':
      main()
  
subject: check for peru file before running