@prefix :      <http://a.example/> .
@prefix sh:    <http://www.w3.org/ns/shacl/core#> .
@prefix xsd:   <http://www.w3.org/2001/XMLSchema#> .

<S>     sh:group     [ sh:maxCount  1 ;
                       sh:minCount  0 ;
                       sh:property  [ a             sh:PropertyConstraint ;
                                      sh:maxCount   1 ;
                                      sh:minCount   1 ;
                                      sh:predicate  :b
                                    ] ;
                       sh:property  [ a             sh:PropertyConstraint ;
                                      sh:maxCount   1 ;
                                      sh:minCount   1 ;
                                      sh:predicate  :a
                                    ]
                     ] ;
        sh:property  [ a             sh:PropertyConstraint ;
                       sh:maxCount   1 ;
                       sh:minCount   1 ;
                       sh:predicate  :c
                     ] ;
        sh:schema    [ a  sh:Schema ] .