Refactor the code to copy the subst in unify_variable
Refactor the code to copy the subst in unify_variable

eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog,eliben/code-for-blog