The '''Yellowstone sequence''', also called the '''Yellowstone permutation''', is defined as:

For n <= 3,

    a(n) = n

For n >= 4,

    a(n) = the smallest number not already in sequence such that a(n) is relatively prime to a(n-1) and 
           is not relatively prime to a(n-2).


The sequence is a permutation of the natural numbers, and gets its name from what its authors felt was a spiking, geyser like appearance of a plot of the sequence.


;Example:
a(4) is 4 because 4 is the smallest number following 1, 2, 3 in the sequence that is relatively prime to the entry before it (3), and is not relatively prime to the number two entries before it (2).


;Task
: Find and show as output the first &nbsp;'''30'''&nbsp; Yellowstone numbers.


;Extra
:  Demonstrate how to plot, with x = n and y coordinate a(n), the first 100 Yellowstone numbers.


;Related tasks:
:* &nbsp; [https://rosettacode.org/wiki/Greatest_common_divisor Greatest common divisor].
:* &nbsp; [https://rosettacode.org/wiki/Plot_coordinate_pairs Plot coordinate pairs].
:* &nbsp; [[EKG sequence convergence]]


;See also:
:* &nbsp; The OEIS entry: &nbsp; [https://oeis.org/A098550 A098550 The Yellowstone permutation].
:* &nbsp; Applegate et al, 2015: The Yellowstone Permutation [https://arxiv.org/abs/1501.01669].
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