@prefix skos:	<http://www.w3.org/2004/02/skos/core#> .
@prefix isac:	<http://webisa.webdatacommons.org/concept/> .
isac:_astigmatism_	skos:broader	isac:refractive_problem_ .
isac:_myopia_	skos:broader	isac:refractive_problem_ .
isac:_farsightedness_	skos:broader	isac:refractive_problem_ .