The function GetSystem(system) returns all units in a system of units.

For an existing system system, the call AddSystem(system, GetSystem(system)) recreates an equivalent system. The calling sequence AddSystem(system, check, GetSystem(system), new_unit) redefines the system by including new_unit and ensuring that new_unit does not have the same dimension as any of the existing units in system.

$\mathrm{with}\left(\mathrm{Units}\right)\:$

Automatically loading the Units[Simple] subpackage

$\mathrm{GetSystem}\left('\mathrm{CGS}'\right)$

${\mathrm{barye}}_{\mathrm{standard}},\frac{{\mathrm{erg}}_{\mathrm{standard}}}{{\mathrm{second}}_{\mathrm{SI}}},{\mathrm{dollar}}_{\mathrm{US}},{\mathrm{centimeter}}_{\mathrm{SI}}^2,{\mathrm{gram}}_{\mathrm{SI}},{\mathrm{dyne}}_{\mathrm{standard}},{\mathrm{gal}}_{\mathrm{standard}},{\mathrm{dyne}}_{\mathrm{standard}}{\mathrm{centimeter}}_{\mathrm{SI}}\left(\mathrm{radius}\right),{\mathrm{centimeter}}_{\mathrm{SI}},{\mathrm{centimeter}}_{\mathrm{SI}}^3,{\mathrm{second}}_{\mathrm{SI}},{\mathrm{kelvin}}_{\mathrm{SI}},{\mathrm{erg}}_{\mathrm{standard}},{\mathrm{steradian}}_{\mathrm{SI}},{\mathrm{radian}}_{\mathrm{SI}},{\mathrm{kyne}}_{\mathrm{standard}}$

$\mathrm{AddSystem}\left('\mathrm{FHF}'\,'\mathrm{check}'\,'\mathrm{furlong}'\,'\mathrm{hundredweight}\left[\mathrm{long}\right]'\,'\mathrm{fortnight}'\right)$

$\mathrm{GetSystem}\left('\mathrm{FHF}'\right)$

${\mathrm{hundredweight}}_{\mathrm{long}},{\mathrm{furlong}}_{\mathrm{standard}},{\mathrm{fortnight}}_{\mathrm{standard}}$

$\mathrm{AddSystem}\left('\mathrm{MKSA}'\,\mathrm{GetSystem}\left('\mathrm{MKS}'\right)\,'\mathrm{ampere}'\right)$

$\mathrm{infolevel}\left[\mathrm{Units}\right]≔1\:$

$\mathrm{convert}\left(96485.309\,'\mathrm{system}'\,'\mathrm{electron}'\,'\mathrm{MKS}'\right)$

Error, (in `convert/system`) cannot represent the given unit in the system `MKS`

$\mathrm{convert}\left(96485.309\,'\mathrm{system}'\,'\mathrm{electron}'\,'\mathrm{MKSA}'\right)$

convert/system:   "converting to the units A*s"

$1.545865063\times {10}^{\mathrm{-14}}$