IS A

From ASCEND

The IS_A statement is used to declare a new variable or sub-MODEL in your MODEL.

x IS_A distance; (* this is an 'atomic' variable: just
    a single numerical value that can be solved for *)
v IS_A vec3; (* vec3 is a sub-model containing x,y,z components *)
v.x = x; (* once variables are declared, you can use them
    in relations involving other variables, possibly from your
    sum-models *)

See also WILL_BE, IS_REFINED_TO, ALIASES, ARE_THE_SAME.