Covariance

**Return Type**`A::return_type`The type of

*C*(,) (see definition above).**Argument Type**`A::argument_type`The type of location

**u**(see definition above).

A | A type that is a model of Covariance | |

a | an object of type A | |

U | A type that is a model of Location | |

u1,u2 | two objects of type U | |

T | A type that represents a real number (e.g. float, double) |

**Compute covariance**`a(u1,u2)`Return type: `T`Semantics: returns the covariance *C*(`u1,u2`).

`Spherical_covariance``Exponential_covariance`