PK_LINE_sf_t

struct PK_LINE_sf_s
    {
    PK_AXIS1_sf_t basis_set; --- a cartesian point and a direction
    };

typedef struct PK_LINE_sf_s PK_LINE_sf_t;

Used in:

PK_LINE_ask
PK_LINE_create

This is the standard form for an unbounded straight line which passes through
the given 'location' in the given 'direction'.

The parameterisation of the line is as follows:

    P=basis_set.location
    V=basis_set.direction

    P( t ) = P + t*V