Fix ref issue.
Fix ref issue.

Closes #57.

DuncanAForbes/substance,abhaga/substance,TypesetIO/substance,abhaga/substance,andene/substance,podviaznikov/substance,substance/substance,GerHobbelt/substance,DuncanAForbes/substance,TypesetIO/substance,jacwright/substance,andene/substance,GerHobbelt/substance,michael/substance-1,michael/substance-1,jacwright/substance,stencila/substance,podviaznikov/substance,substance/substance