Multiplexing STS$-1$ multiple data streams, called tributaries, plays an important role in SONET. A $3$:$1$ multiplexer multiplexes three input STS$-1$ tributaries onto one output STS$-3$ stream. This multiplexing is done byte for byte. That is$,$ the first three output bytes are the first bytes of tributaries $1,2,$ and $3,$ respectively. the next three output bytes are the second bytes of tributaries $1,2,$ and $3,$ respectively, and so on$.$ Write a program that simulates this $3$:$1$ multiplexer. Your program should consist of five processes. The main process creates four processes, one each for the three STS$-1$ tributaries and one for the multiplexer. Each tributary process reads in an STS$-1$ frame from an input file as a sequence of $810$ bytes. They send their frames $($byte by byte$)$ to the multiplexer process. The multiplexer process receives these bytes and outputs an STS$-3$ frame $($byte by byte$)$ by writing it to standard output. Use pipes for communication among processes.