MO::Basis::LCAO#shells should return a collection of BasisSet::LCAO::Gaussian::Shells
MO::Basis::LCAO::Gaussian#overlap should return a matrix
RuPHY::Math::Matrix#diagonal_elements
Define all_be_within matcher
Specs for MO::Basis::LCAO::Gaussian#overlap
Initialize shells
Spec for BasisSet::LCAO::Gaussian#shells

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BasisSet::LCAO::Gaussian#shells takes element

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