class_docstr: UnitSystem represents a coherent set of units.
  
  A unit system is basically a dimension system with notions of scales. Many
  of the methods are defined in the same way.
  
  It is much better if all base units have a symbol.
class_docstr_tok: ["UnitSystem","represents","a","coherent","set","of","units",".","A","unit","system","is","basically","a","dimension","system","with","notions","of","scales",".","Many","of","the","methods","are","defined","in","the","same","way",".","It","is","much","better","if","all","base","units","have","a","symbol","."]
docstr: Extend the current system into a new one.
  
  Take the base and normal units of the current system to merge
  them to the base and normal units given in argument.
  If not provided, name and description are overridden by empty strings.
docstr_tok: ["Extend","the","current","system","into","a","new","one",".","Take","the","base","and","normal","units","of","the","current","system","to","merge","them","to","the","base","and","normal","units","given","in","argument",".","If","not","provided",",","name","and","description","are","overridden","by","empty","strings","."]
code: def extend(
      self,
      base,
      units=(),
      name="",
      description="",
      dimension_system=None,
      derived_units: tDict[Dimension, Quantity] = {},
  ):
      """Extend the current system into a new one.
  
      Take the base and normal units of the current system to merge
      them to the base and normal units given in argument.
      If not provided, name and description are overridden by empty strings.
      """
  
      base = self._base_units + tuple(base)
      units = self._units + tuple(units)
  
      return UnitSystem(
          base,
          units,
          name,
          description,
          dimension_system,
          {**self._derived_units, **derived_units},
      )
  
imports: from typing import Dict, Set
  from sympy.core.add import Add
  from sympy.core.function import Derivative, Function
  from sympy.core.mul import Mul
  from sympy.core.power import Pow
  from sympy.core.singleton import S
  from sympy.physics.units.dimensions import _QuantityMapper
  from sympy.physics.units.quantities import Quantity
  from .dimensions import Dimension
label_desc: Use image node_id 4 for calling the UnitSystem obj's underlying member method code with example usage: obj.extend(base, units, name, description, dimension_system, derived_units) and returns: UnitSystem