  Quantum algorithms can be analyzed in a query model to compute Boolean
functions. Function input is provided in a black box, and the aim is to compute
the function value using as few queries to the black box as possible. A
repetition code is an error detection scheme that repeats each bit of the
original message r times. After a message with redundant bits is transmitted
via a communication channel, it must be verified. If the received message
consists of r-size blocks of equal bits, the conclusion is that there were no
errors. The verification procedure can be interpreted as an application of a
query algorithm, where input is a message to be checked. Classically, for N-bit
message, values of all N variables must be queried. We present an exact quantum
algorithm that uses only N/2 queries.
