<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def read_len( f ):
 """Read a 'LEN' file and return a mapping from chromosome to length"""

    mapping = dict()
    for line in f:
        fields = line.split()
        mapping[ fields[0] ] = int( fields[1] )
    return mapping