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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def clean_missing_exponent(pst_filename,clean_filename="clean.pst"):
 """fixes the issue where some terrible fortran program may have written a floating point format without the 'e' - like 1.0-3, really?! Parameters pst_filename : str the pest control file clean_filename : str the new pest control file to write. Default is "clean.pst" Returns ------- None """

    lines = []
    with open(pst_filename,'r') as f:
        for line in f:
            line = line.lower().strip()
            if '+' in line:
                raw = line.split('+')
                for i,r in enumerate(raw[:-1]):
                    if r[-1] != 'e':
                        r = r + 'e'
                    raw[i] = r
                lines.append('+'.join(raw))
            else:
                lines.append(line)
    with open(clean_filename,'w') as f:
        for line in lines:
            f.write(line+'\n')