Validate complete absence of bios

validation-for-ya.rq

# Explanation:
# ASK returns 'true' if there are any results
# The inner query says:
# Get agents and (if there) bio:event
# Count agents and bios, and check if there are agents present, but no bios

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX bio: <http://purl.org/vocab/bio/0.1/>
ASK {
    SELECT (count(?x) as ?agents) (count(?y) as ?bios)
    { 
        ?x a foaf:Agent
        OPTIONAL { ?x bio:event ?y }
    } HAVING (?agents > 0 && ?bios = 0)
}