paramete.spad line 63 [edit on github]
ParametricSpaceCurve is used for plotting parametric space curves in affine 3-space.
coordinate(c, i) returns a coordinate function of c using 1-based indexing according to i. This indicates what the function for the coordinate component, i, of the space curve is.
curve(c1, c2, c3) creates a space curve from 3 component functions c1, c2, and c3.