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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 package_version(self, prefix=None, name=None, pkg=None, build=False):
 """Get installed package version in a given env."""

        package_versions = {}

        if name and prefix:
            raise TypeError("Exactly one of 'name' or 'prefix' is required.")

        if name:
            prefix = self.get_prefix_envname(name)

        if self.environment_exists(prefix=prefix):

            for package in self.linked(prefix):
                if pkg in package:
                    n, v, b = self.split_canonical_name(package)
                    if build:
                        package_versions[n] = '{0}={1}'.format(v, b)
                    else:
                        package_versions[n] = v

        return package_versions.get(pkg)