Basis Variable and Equations
```
1. Enter var-defined, var = P1Y
2c. deleting tuple (= P1 (TUPLE (X P1X) (Y P1Y)))
2d. deleting eqn   (= P1X (X P1))
2d. deleting eqn   (= P1Y (Y P1))
4. exit, vars (P1 P1Y)
1. Enter var-defined, var = LENGTH
4. exit, vars (LENGTH)
1. Enter var-defined, var = THETA
2a. solved eqn     (= SLOPE (TAN THETA))
2a. solved eqn     (= PHI (- (/ PI 2.0) THETA))
2a. solved eqn     (= DELTAY (* LENGTH (SIN THETA)))
2a. solved eqn     (= DELTAX (* LENGTH (COS THETA)))
3.  repeating step 2.
2a. solved eqn     (= DELTAY (- P2Y P1Y))
giving     (= P2Y (+ DELTAY P1Y))
2b. deleting eqn   (= SLOPE (/ DELTAY (FLOAT DELTAX)))
2b. deleting eqn   (= SLOPE (/ 1.0 (TAN PHI)))
2b. deleting eqn   (= LENGTH (SQRT (+ (EXPT DELTAX 2)
(EXPT DELTAY 2))))
2b. deleting eqn   (= THETA (ATAN DELTAY DELTAX))
2b. deleting eqn   (= PHI (ATAN DELTAX DELTAY))
2b. deleting eqn   (= DELTAY (* LENGTH (COS PHI)))
2b. deleting eqn   (= DELTAX (* LENGTH (SIN PHI)))
3.  repeating step 2.
2c. deleting tuple (= P2 (TUPLE (X P2X) (Y P2Y)))
2d. deleting eqn   (= P2X (X P2))
2d. deleting eqn   (= P2Y (Y P2))
4. exit, vars (P2 P2Y DELTAX DELTAY PHI SLOPE THETA)
1. Enter var-defined, var = P2X
2a. solved eqn     (= DELTAX (- P2X P1X))
giving     (= P1X (- P2X DELTAX))
3.  repeating step 2.
4. exit, vars (P1X P2X)
```