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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 validate_vars(env):
 """Validate the PCH and PCHSTOP construction variables."""

    if 'PCH' in env and env['PCH']:
        if 'PCHSTOP' not in env:
            raise SCons.Errors.UserError("The PCHSTOP construction must be defined if PCH is defined.")
        if not SCons.Util.is_String(env['PCHSTOP']):
            raise SCons.Errors.UserError("The PCHSTOP construction variable must be a string: %r"%env['PCHSTOP'])