The MEXTRAM code here has been modified from the code distributed by its
authors.  This file documents the changes needed:

  - Xyce-specific attributes were added to the "module" line for ease of
    implementation in Xyce.  bjt504 becomes Xyce Q device level 504,
    and bjt504t becomes Xyce Q device level 505.  bjtd504 and bjtd504t are
    not used in Xyce.

  - An analog function "trunc_ev" was added for voltage-limiting of
    some junction voltages, and (very Xyce-specific) "$limit" uses
    were hacked into evaluate.inc.  This dramatically improves MEXTRAM
    convergence on CMC QA tests.  Without these limiters imposed, MEXTRAM
    fails to converge the operating point on some of the tests.

  - variables.inc was modified to add a "tempx" variable, and tscaling.inc
    was modified to use this variable as a temporary instead of the
    "x" variable.  x is now used only in main module scope, and tempx is
    used only in initial_instance scope.  This keeps both of those uses
    separate, and lets ADMS flag them as "local" variables instead of
    "global_instance" variables.

  - An attribute was added to the MULT parameter to flag it as being accepted
    as both instance and model parameter.  If given in the model and not on
    the instance line, the model value is used.  If given on the instance line,
    the value there overrides any in the model.

  - The parameter "M" was made an alias for "MULT".

  - Various Xyce-specific ifdefs have been added to frontdef.inc and
    opinfo.inc to work around limitations of Xyce/ADMS.  These are to
    address the lack of support for port current accessors such as
    I(<e>) to get the emitter current.

  - The OPP macro has been modified for Xyce to enable proper
    expansion of quoted strings, and the use of this macro changed to
    quote the units and descriptions.
