preferred_name: [3-O-methyl-alpha-D-mannosyl-(1->4)](n)-1-O,3-O-dimethyl-alpha-D-mannose + GDP-alpha-D-mannose => alpha-D-mannosyl-(1->4)-[3-O-methyl-alpha-D-mannosyl-(1->4)](n)-1-O,3-O-dimethyl-alpha-D-mannose + GDP + H(+)