The round unknot is a ribbon knot

Let $\gamma(t)=(\cos 2\pi t,\sin 2\pi t,0,0)\in S^3\subset\mathbb R^4$. Then $\gamma$ parametrises a knot (it is $C^\infty$, $1$-periodic, injective on $[0,1)$, with nonzero velocity) and

i.e. it bounds an immersed disc in $S^3$ with only ribbon singularities (here an embedded disc suffices).

This sanity check shows that the definitions of knots and ribbon discs used in this mission are satisfiable.

Formalization Note IsKnot γ ∧ IsRibbon γ for the explicit $\gamma$, written with !₂[...].