What is the literary, symbolic, and thematic purpose of the paper-knife in Sartre's No Exit?

The paper knife represents the futility of any of the
character's efforts to cut through what each is, or to terminate their existence in the
room. After Garcin in his frustration shouts for the door to open, and it does, he then
is paralyzed with the fear of the unknown and cannot leave.  He stays because Inez knows
what it is like to be a coward.  Likewise, Estella and Inez do not act. Garcin tells
Inez,

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Yes. You, anyhow, know what
it means to be a
coward. 

Shortly thereafter,
Inez remarks, "We're--inseparable!"  They understand each other; they see each other for
what they truly are.  The verity of Inez'a statement is emphasized by Estella's attempt
to stab Inez and kill her with the paper knife, and then her desperate attempt to kill
herself.  Laughing, Inez sums up their situation in her realization of its
finality:

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Dead!  Dead!  Dead!  Knives, poison, ropes--all
are useless.  It has happened already, do you understand?  Once and
for all. So here we are,
forever. 

Garcin stops his
laughing, slumps onto his sofa:  "Well, well, let's get on with it....."  Certainly, the
paper knife, symbolic of the futility of the three characters to change themselves and
dissemble, underscores the theme of Jean-Paul Sartre:  "Hell is---other
people!"