Heap cells contain a number of entries, each of which is either a scheme
object of type
SCM or a raw C value of type
Which of the cell entries contain Scheme objects and which contain raw C
values is determined by the first entry of the cell, which holds the
cell type information.
For a non-immediate Scheme object x, deliver the content of the first entry of the heap cell referenced by x. This value holds the information about the cell type.
For a non-immediate Scheme object x, write the value t into the first entry of the heap cell referenced by x. The value t must hold a valid cell type.