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http://journals.cambridge.org/action/displayAbstract?aid=248783
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ROBERT ROSEBRUGH,
NICOLETTA SABADINI
and ROBERT F. C. WALTERS
(2004).
Minimisation and minimal realisation in Span(Graph). Mathematical Structures in Computer Science, 14
,
pp 685-714
doi:10.1017/S096012950400430X