A declaration is the mathematical definition of a set of variables. It is the lowest level building block for a model
in `assemblerr`. A declaration consists of the variable names being declared (the identifiers) and their definition. The
`declaration` function allows the specification of a declaration using `R` formulae.
Usage
declaration(...)
Arguments
...
List of R formulae with a single symbol on the left-hand side and a valid R expression on the right