Problems with Longrightarrow and friends

[Martin Ward <Martin.Ward@durham.ac.uk>]

I have done some more experimentation with the \Longrightarrow problem.
\Longrightarrow is constructed from a normal = (which I believe comes from
the text fonts) and a \Rightarrow (from the cmsy font). These are placed
side-to-side without overlapping.

This leads to three potential problems:

(1) With the dc fonts, digitised at 300dpi, the = comes out much
thinner than the cm fonts. This means that it doesn't line up with the
Rightarrow in cmsy.

(2) There is a cmr17 but no cmsy17. At \huge or \Huge sizes, the = comes
>From cmr17 but the Rightarrow comes from a scaled cmsy10. At these sizes,
the symbols no longer match.

(3) The Rightarrow and the = have slightly rounded ends to their horizontal
bars. If these are abutted without overlapping, the result has one or more
missing pixels in the horizontal bars.

A quick fix for (3) is to replace \joinrel in the definition of
\Longrightarrow, \Longleftarrow, Longleftrightarrow, etc. by \overlaprel
which is defined:
	\def\overlaprel{\mathrel{\mkern-5mu}}
\joinrel is defined:
	\def\joinrel{\mathrel{\mkern-3mu}}

I don't have fixes for (1) and (2), can anyone help?

Enclosed is a file which illustrates the problem.

			Martin.

JANET: Martin.Ward@uk.ac.durham    Internet (eg US): Martin.Ward@durham.ac.uk
or if that fails:  Martin.Ward%uk.ac.durham@nsfnet-relay.ac.uk  
or even: Martin.Ward%DURHAM.AC.UK@CUNYVM.CUNY.EDU
BITNET: Martin.Ward%durham.ac.uk@UKACRL UUCP:...!uknet!durham!Martin.Ward

\documentstyle{article}
% Also try with [11pt] and [12pt].
\def\implies{\;\Longrightarrow\;}

\begin{document}

\noindent
\scriptsize scriptsize:
\[\implies\quad=\quad\iff\]
\small small:
\[\implies\quad=\quad\iff\]
\normalsize normalsize:        
\[\implies\quad=\quad\iff\]
\large large: 
\[\implies\quad=\quad\iff\]
\Large Large:
\[\implies\quad=\quad\iff\]
\LARGE LARGE:
\[\implies\quad=\quad\iff\]
\huge huge:
\[\implies\quad=\quad\iff\]
\Huge Huge:
\[\implies\quad=\quad\iff\]


\end{document}