<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _set_seq(self,sequence,start=0):
 """Set the sequence from a start position for the length of the sequence"""

    if start+len(sequence) > self._slen: 
      sys.stderr.write("Error not long enough to add\n")
      sys.exit()
    z = 0
    for i in xrange(start, start+len(sequence)):
      self._set_nt(sequence[z],i)
      z+=1