code: def to_string(self):
          
          def filt(x):
              return '+'+x[0] in PROJ4_PARAMS.keys() and x[1] is not False
  
          items = []
          for k, v in sorted(filter(filt, self.items())):
              items.append(
                  "+" + "=".join(
                      map(str, filter(
                          lambda y: (y or y == 0) and y is not True, (k, v)))))
          return " ".join(items)
docs: Turn a CRS dict into a PROJ.4 string. Mapping keys are tested against
          ``all_proj_keys`` list. Values of ``True`` are omitted, leaving the key
          bare: {'no_defs': True} -> "+no_defs" and items where the value is
          otherwise not a str, int, or float are omitted.
  
          Args:
              crs: A CRS dict as used in Location.
  
          Returns:
              str. The string representation.