code: def findExtname(fimg, extname, extver=None):
      
  
      i = 0
      extnum = None
      for chip in fimg:
          hdr = chip.header
          if 'EXTNAME' in hdr:
              if hdr['EXTNAME'].strip() == extname.upper():
                  if extver is None or hdr['EXTVER'] == extver:
                      extnum = i
                      break
          i += 1
      return extnum
docs: Returns the list number of the extension corresponding to EXTNAME given.