Fix the warning in 8e207729
Add a check for yices on real in nightly-build

new syntax 'L: to introduce a mark
new syntax at t 'L in logical expressions

First version of DEVELOPER.readme, modify it according to your understanding of the current informal guidelines

Warning since 7efae3f8 yices doesn't
work with real since in assoc_mul_div a variable is the denominator.
Yices refuse that.

updated DTD

because it is incompatible with the session system