code: def maxCtxFont(font):
      """Calculate the usMaxContext value for an entire font."""
  
      maxCtx = 0
      for tag in ("GSUB", "GPOS"):
          if tag not in font:
              continue
          table = font[tag].table
          if not table.LookupList:
              continue
          for lookup in table.LookupList.Lookup:
              for st in lookup.SubTable:
                  maxCtx = maxCtxSubtable(
                      maxCtx, tag, lookup.LookupType, st
                  )
      return maxCtx
  
label_desc: Use image node_id 1 for calling a global function with example usage: maxCtxFont(font) and returns: maxCtx