HD 11/8/17. -- Green in Sherpa: Author's Pre-print: green tick author can archive pre-print (ie pre-refereeing) Author's Post-print: green tick author can archive post-print (ie final draft post-refereeing) Publisher's Version/PDF: green tick author can archive publisher's version/PDF General Conditions: Author's pre-print and Author's post-print on author's personal website, institutional repository, arXiv or preprint servers only Publisher copyright must be acknowledged Must link to publisher version Non-commercial use only Authors retain copyright Publisher's version/PDF may be used on author's personal website or employers' website only Authors may have deposit in funding body's archive or funding body's designated repository for public release 6 months after publication or immediately upon payment of fee All PNAS-formatted versions are deposited in PubMed Central for release after 6 months, regardless of funding. -- Full-text added 3.10.17-MM