This function is not yet fully documented. This is a transcript of the text-formatted help.

bbcompare  Compare two black-box operators.
  bbcompare(BB1,BB2) tests whether two operators are identical
  up to rounding-errors. It generates a number of test-vectors
  which it multiplies.

  This is convenient for testing whether an operator is
  implemented correctly. It is often possible to create a full
  matrix for some small examples, which can be used to validate
  an implementation.

  [suc,msg,warn]=bbcompare(BB1,BB2) returns true if the test
  passed and false otherwise. It also creates an message
  describing why the test failed or succeeded. The least
  output is a string with non-fatal warnings.

  bbcompare(BB1,BB2,K) uses K test-vectors. If K is a vector,
  the test is performed as a series of matrix-matrix evaluations
  with the i'th test using K(i) vectors at a time. This is handy
  when testing if matrix-matrix multiplications are correct.

  The comparison is done using 4*sum(K) evaluations:
  right- and left-multiplication with real and complex vectors.

  See also bbtauest.