LEVASS ( assemb, result, ifail )
================================

  Level assembly.

Receives:
  KI_tag_assembly          *assemb         --- assembly to level

Returns:
  KI_tag_assembly          *result         --- new levelled assembly
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_anon_sub_part          instance of anonymous sub-part of stored part

Description:
  LEVASS creates a new assembly which contains, for each path from 'assemb'
  to a body contained in it, an instance of that body with a transform equal to
  the product of the transforms attached to instances along the path.

  LEVASS will always create a new assembly, even if 'assemb' is already level,
  or empty. 'assemb' is unchanged.

  If 'assemb' is stored (ENSTST) and has a body as an anonymous true-sub-part
  the assembly created by LEVASS would cause 'assemb' to become unisolated. In
  this case KI_anon_sub_part is returned in 'ifail'.