Multiple changes.

Restored waiting period between retries of read_expression and
write_expression.

Shortened waiting period.

Added debug messages in p0.c, p1.c, and p2.c.

Completed function multiply_divide_prime in p1.c. Replaced
multiply_divide with it.
Implemented function add_subtract_prime. Replaced add_subtract with it.

function add_subtract_prime
This function calculates the additions and subtractions found in
the linear algebraic expression expression and overwrites it with
the resulting expression.
Removed obsolete code.

Removed multiply_divide from p1.c, and add_subtract from p2.c.
Improved debug messages.
Commented out debug messages.
Multiple changes.

Copied multiply_divide_prime and add_subtract_prime from files
p1.c and p2.c to pipes p1.c and p2.c.

Removed multiply_divide and add_subtract from pipes p1.c and p2.c.

More minor changes.