A (non-empty) stack of summary components.
A stack is used to represent where data is read from (input) or where it
is written to (output). For example, an input stack
[Field f, Argument 0]
means that data is read from field
f from the
0th argument, while an
[Field g, Return] means that data is written to the field
g of the returned object.
Gets the bottom element of this stack.
Holds if this stack contains summary component
Gets the stack obtained by dropping the first
Gets the head of this stack.
Gets the length of this stack.
Gets the tail of this stack, if any.
Gets a textual representation of this stack.