added pointer to today’s
added the twso original references giving the F-theory picture:
Paul Aspinwall, Mark Gross, The Heterotic String on a K3 Surface, Phys. Lett. B387 (1996) 735-742 (arXiv:hep-th/9605131)
Paul Aspinwall, David Morrison, Point-like Instantons on K3 Orbifolds, Nucl. Phys. B503 (1997) 533-564 (arXiv:hep-th/9705104)
added also comment on and pointer to
now some content in place
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