docs: def build(self, builder):
          """Build XML by appending to builder"""
          params = dict(OID=self.oid)
          params["mdsol:ProjectType"] = self.project_type
  
          builder.start("Study", params)
  
          # Ask children
          if self.global_variables is not None:
              self.global_variables.build(builder)
  
          if self.basic_definitions is not None:
              self.basic_definitions.build(builder)
  
          if self.metadata_version is not None:
              self.metadata_version.build(builder)
  
          builder.end("Study")