| http://www.w3.org/ns/prov#value | - A directed graph is a graph where edges are one-way, so a directed edge from nodes $X$ to $Y$ only allows information to pass from $X$ to $Y$ and not from $Y$ to $X$. # Specify a directed graph by handG <- graph.formula(1-+2,1-+3,2-+4,2-+5,3-+6,5-+7,7-+8,8-+9,9+-7, 9-+10, 6-+9,1-+5,5-+20,3-+9,10-+11,10-+15,11-+12,11-+13,13-+14, 14-+15, 15-+11,4-+8,15-+16,15-+17,17-+18,18-+4,17-+19,19-+
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