This function implements the original definition of the three-dimensional Kolmogorov-Smirnov test by Peacock (1983). This test is not the widely used Fasano-Franceschini test (1987). The latter is a variant the Peacock test.
x is the object representing the first sample. x should be able to be converted a matrix, where each row represents a sample point. If the object cannot be converted to a matrix, the function will stop and throw an error message. Pleaste note that only the first three columns of the matrix will be used, and the rest columns are just ingored.
Similar to x, y is the object representing the second sample.
the value of the test statistic
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