Add the Eliminate arrow to linear.
Add functions to linear for creating Eliminate arrows.
Implement substituteVar Eliminate for all linearui objects.

Implement the substituteVar method for the linearui objects defined in
natural.py.

Also, define the linearui Eliminate transition and its translation into
a linear arrow.
Fix == typo.
Implement methods for creating linearui Eliminate arrows.
Implement moving quantifiers to the outside of conjs.

Implement the requisite arrows in linear.
Implement the requisite arrows in linearui, along with their
translations into linear.
Test and yix bugs regarding moving quantifiers outside conjs.
Implement forwardAssociateIn and test and fix its bugs.
Test and debug eliminating a variable.
Implement removal of empty linearui quantifiers, test and fix bugs.
Test importToPar and fix bugs.
Simplify the proof in induction.
Remove the
Fix bugs.
Merge branch 'dev' into linearui-apply
Implement linearui Forget and Apply.
Test linearui apply and fix bugs.
Implement linearui remove unit.
Test linearui remove unit and fix bugs.
Implement methods to create linearui singleton and unsingleton arrows.