What does a dot over $\dot{D}^2$ mean in topology?

Question

Reading http://projecteuclid.org/euclid.jdg/1214437136, I don't want to guess at the meaning of the overdot on page 1 $$\dot{H} = ( D^2 \times \dot{D}^2, \partial D^2 \times \dot{D}^2)$$.

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J. Differential Geom. Volume 17, Number 3 (1982), 357-453. The topology of four-dimensional manifolds

Michael Hartley Freedman