Robuta

https://leanprover-community.github.io/mathlib4_docs/Mathlib/Geometry/Euclidean/Angle/Unoriented/RightAngle.html Mathlib.Geometry.Euclidean.Angle.Unoriented.RightAngle mathlibgeometryeuclideanangle https://leanprover-community.github.io/mathlib4_docs/Mathlib/Geometry/Euclidean/Angle/Unoriented/Basic.html Mathlib.Geometry.Euclidean.Angle.Unoriented.Basic mathlibgeometryeuclideananglebasic https://annals.math.princeton.edu/1985/121-3/p07 Generic regularity of unoriented two-dimensional area minimizing surfaces | Annals of Mathematics two dimensionalgenericregularity