shaft 10 a lastly a construction line segment 320 connects pivot shafts 10 b to pivot shaft 10 a and another construction line segment 330 connects pivot shaft 10 b to point 20 af point 20 af indicates the phantom location of pivot shaft 20 a with respect to element 10 in the closed form the particular geometric arrangement of the first second and third connection points on the common structural element are then substantially defined as follows the first connection point indicated by pivot shaft 10 a is at the intersection of lines 110 and 140 the third connection point indicated by magnet 10 c is at a convenient position on line 210 the second connection point indicated by pivot shall 10 b is at a particular position on line 220 relative to magnet 10 c such that the length of line segment 320 is substantially equal to the length of line segment 330 in the preferred embodiment illustrated in fig4 the end edge of element 10 near magnet 10 c is substantially parallel to line 120 and offset from line 120 towards center point 200 by a distance s which is greater than or equal to a distance t between line.