Corrected handling of Lit, by introducing also Var.

For unsigned int i, considered as literal, the negation "-i" doesn't
work in general!
Corrected reading of variable-number.

Plus editorial improvements, using "x" for literals and "v" for variables.
And using "branching_literal" instead of "get_variable".
And further corrections regarding use of Lit and Var.
Introduced constants for return-codes.
Defending against Lit=char.
Renamed: reduce -> assign, reverse -> unassign.