Added a parent property to EventNode.
Added a parent property to EventNode.

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