About: http://webisa.webdatacommons.org/336938940

Property	Value
http://www.w3.org/1999/02/22-rdf-syntax-ns#type	http://www.w3.org/ns/prov#Entity
http://www.w3.org/ns/prov#value	@Dan: while the general philosophy of your answer is correct, one of the statements you make is not: It is not true that only $(-1)$-curves can be blown down.
http://www.w3.org/ns/prov#wasQuotedFrom	mathoverflow.net