See section 12.5.1 for lines in space.
The line command returns and draws a directed line given one of the following types of arguments:
Warning: To draw a line with an additional argument for color (such as color = blue), this argument must be the third argument. In particular, for a list of two points to specify a line in this command, the list must be turned into a sequence, such as with op. For example, given a list L of two points (possibly the result of a different command) which determines a line, to draw the line blue enter line(op(L),color=blue); entering line(L,color=blue) results in an error.