code: def maxCtxSubtable(maxCtx, tag, lookupType, st):
      
  
      # single positioning, single / multiple substitution
      if (tag == 'GPOS' and lookupType == 1) or (
          tag == 'GSUB' and lookupType in (1, 2, 3)):
          maxCtx = max(maxCtx, 1)
  
      # pair positioning
      elif tag == 'GPOS' and lookupType == 2:
          maxCtx = max(maxCtx, 2)
  
      # ligatures
      elif tag == 'GSUB' and lookupType == 4:
          for ligatures in st.ligatures.values():
              for ligature in ligatures:
                  maxCtx = max(maxCtx, ligature.CompCount)
  
      # context
      elif (tag == 'GPOS' and lookupType == 7) or (
            tag == 'GSUB' and lookupType == 5):
          maxCtx = maxCtxContextualSubtable(
              maxCtx, st, 'Pos' if tag == 'GPOS' else 'Sub')
  
      # chained context
      elif (tag == 'GPOS' and lookupType == 8) or (
            tag == 'GSUB' and lookupType == 6):
          maxCtx = maxCtxContextualSubtable(
              maxCtx, st, 'Pos' if tag == 'GPOS' else 'Sub', 'Chain')
  
      # extensions
      elif (tag == 'GPOS' and lookupType == 9) or (
            tag == 'GSUB' and lookupType == 7):
          maxCtx = maxCtxSubtable(
              maxCtx, tag, st.ExtensionLookupType, st.ExtSubTable)
  
      # reverse-chained context
      elif tag == 'GSUB' and lookupType == 8:
          maxCtx = maxCtxContextualRule(maxCtx, st, 'Reverse')
  
      return maxCtx
docs: Calculate usMaxContext based on a single lookup table (and an existing
      max value).