The 2H polytype is pleochroic, meaning that the mineral changes colors ( in this case yellow and light yellow ) when viewed at different angles under a polarizing petrographic microscope.
12.
The procedure inst ( \ sigma ) specializes the polytype \ sigma by copying the term and replacing the bound type variables consistently by new monotype variables .'newvar'produces a new monotype variable.
13.
Starting with a polytype \ forall \ alpha . \ alpha, the specialization could either replace the body by another quantified variable, actually a rename or by some type constant ( including the function type ) which may or may not have parameters filled either with monotypes or quantified type variables.
14.
While this rule works well for any monotype used as replacement, it fails when a polytype, say \ forall \ beta . \ beta is tried as a replacement, resulting in the non-syntactical type \ forall \ beta . \ beta \ rightarrow \ forall \ beta . \ beta.
