In a spacious art gallery, a large, square pillar stands in the middle of the main exhibition hall. The pillar is a structural element and also serves as a display surface for various paintings. Four visitors - Neela, Kofi, Leandro, and Fatima - are wandering through the exhibition. Neela and Kofi are standing on opposite sides of the pillar, both closely examining the paintings on their respective sides. Leandro is walking around the pillar in a clockwise direction from Neela's perspective. Meanwhile, Fatima is standing with her back against the wall in a corner of the gallery, from where she has a view of two adjacent sides of the pillar.

Leandro, being an avid fan of abstract art, is moving slowly around the pillar, pausing at each painting to contemplate its intricacies. As he starts his walk from the side where Neela is standing, he is momentarily visible to Fatima before disappearing from her view behind the pillar. Given his slow pace, Leandro only observes the artwork on the side he is facing and does not look around the corner of the pillar.

As Leandro progresses halfway around the pillar, he becomes visible to Kofi. Simultaneously, Fatima, who is still observing the room, sees Leandro come into view again after having lost sight of him when he was behind the pillar from her perspective. Meanwhile, Neela, who has been engrossed in the artwork, decides to take a few steps back to better appreciate the painting, inadvertently positioning herself diagonally across the pillar from her original spot.

Considering the positions of Neela, Kofi, Leandro, and Fatima, the structure of the pillar, and the behavior and movements of these individuals, what can each person see, and what is the sequence of interactions between them?
As we examine the location and movements of Neela, Kofi, Leandro, and Fatima concerning the square pillar and the gallery space, we can infer several outcomes based on spatial reasoning.

Leandro begins his journey around the pillar from where Neela stands. Given that Fatima has a vantage point of two adjacent sides of the pillar and that Leandro starts from Neela's position, visible to Fatima, she initially sees Leandro passing by before he moves out of her line of sight behind the pillar. Leandro remains out of Fatima's sight until he reaches the midway point of his walk around the pillar.

When Leandro reaches the side of the pillar opposite Neela, he enters Kofi's field of vision. At this exact moment, Fatima, from her corner spot, can now see Leandro once again as he has reached the side of the pillar thats adjacent to one of the sides she can see. Fatima will not be able to see Neela as the latter is blocked by the pillar when moving diagonally from her starting position. However, Fatima continues to have a partial view of Kofi, who is on the opposite side of Leandro and, by extension, also opposite Neela.

Neela decides to step back to view her painting from a different angle, and as she does so, she steps diagonally relative to her original position. Since she has moved along the diagonal, she will now be able to catch a glimpse of Kofi, who was formerly on the opposite side of the pillar.

Kofi, now visible to Neela after she repositions herself, continues observing the paintings on his side of the pillar. He has seen Leandro pass by but cannot see either Neela, who is diagonally opposite and blocked from his direct line of sight by the corner of the pillar, or Fatima, who is behind him and out of his line of sight.

Thus, the sequence of interactions and observations is as follows:
1. Fatima observes Leandro during the initial stage of his movement and loses sight of him when he moves behind the pillar.
2. Leandro comes into Kofi's field of vision as he crosses the halfway point of the pillar.
3. At the same time, Leandro re-enters Fatima's field of vision as he moves into the space of the sides she can observe.
4. Neela, upon repositioning diagonally, can now see Kofi, who was previously not in her line of sight.

Each person's view is obstructed or aided by the square pillar based on their position and movements within the art gallery. Neela sees the artwork and eventually Kofi, Kofi sees the artwork and Leandro, Leandro sees only the side of the pillar he is facing as he walks around, and Fatima can observe Leandro initially, then loses sight of him, but sees him again, while maintaining partial visibility of Kofi throughout.