docs: def is_merc_projection(srs):
      """ Return true if the map projection matches that used by VEarth, Google, OSM, etc.
      
          Is currently necessary for zoom-level shorthand for scale-denominator.
      """
      if srs.lower() == '+init=epsg:900913':
          return True
  
      # observed
      srs = dict([p.split('=') for p in srs.split() if '=' in p])
      
      # expected
      # note, common optional modifiers like +no_defs, +over, and +wkt
      # are not pairs and should not prevent matching
      gym = '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null'
      gym = dict([p.split('=') for p in gym.split() if '=' in p])
          
      for p in gym:
          if srs.get(p, None) != gym.get(p, None):
              return False
  
      return True