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Check the feasibility of the candidate pair {n, m}. A candidate pair is
syntactically feasible iff all k-look-ahead rules hold. These look ahead
rules check adjacency relation of the mapping. If an edge is mapped in g1
it should also be mapped in g2 and vise-versa (0-look-ahead). If an edge
in g1 is unmapped but the edge is adjacent to an another mapped vertex
(terminal) then the number of such edges should be equal in g1 compared
to g2 (1-look-ahead). If the edge is unmapped and non-terminal then the
number of such edges should be equal in g1 compared to g2 (2-look-ahead).