struct.f
In IsForth a structure is defined as a constant size. Each element added to the structure increases the overall size of the structure until the structure is closed. At this time the constant is set and can itself be used as an element within other structures.
This file contains words to create structures and to populate said structures with various element types (byte, word, dword sized items etc). These words do not allocate any memory, they merely create a constant to return the size of the structure.
This word (pronounced "struct colon") is used to create a new structure. At the time of creation its size is not known so the value of this structure (actually a constant) will be set once the structure size is known.
This word leaves a 0 at the top of the parameter stack which is the current size of the structure. It also leaves the address of the body of the constant so that it can be back patched later.