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Escape further checks if matrix entries are not all in the set {-1,0,1}, and apply rowSum instead of colSum within Heller-Tompkins check.
There may be other issues in the function that need more comprehensive testing, but I've tested this on a few cases that gave incorrect results before:

is_totally_unimodular(diag(2,1))
is_totally_unimodular(matrix(c(1,1,1,-1),nrow=2))
is_totally_unimodular(matrix(c(0,1,1,1,0,1,1,1,0), nrow=3))
is_totally_unimodular(matrix(c(1,1,0,0,1,0,1,0,1,0,0,1,1,1,1,1), nrow=4))

Escape further checks if matrix entries are not all in the set {-1,0,1}, and apply rowSum instead of colSum within Heller-Tompkins check.
There may be other issues in the function that need more comprehensive testing, but I've tested this on a few cases that gave incorrect results before:

is_totally_unimodular(diag(2,1))
is_totally_unimodular(matrix(c(1,1,1,-1),nrow=2))
is_totally_unimodular(matrix(c(0,1,1,1,0,1,1,1,0), nrow=3))
is_totally_unimodular(matrix(c(1,1,0,0,1,0,1,0,1,0,0,1,1,1,1,1), nrow=4))
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