value index :a    # store `value' in array linked to top of stack `a', with index `index'
      index ;a    # push a[index] on (main) stack

[z 1- :a z 0 !=F]dsFx    # or I could just write such a function for you :)

[         # Everything is wrapped in one big macro
  [         # An inner macro for our *then* part
              # <-- Stuff to execute if a==b here
  2Q          # Then quit the inner and outer macro
]sE       # `E' is for Execution register ;)
la lb =E  # if a==b, execute E
          # if E is executed, it will quit the whole macro, so the rest is never reached:
          # <-- Stuff to execute if a!=b here
]x        # End macro; Execute

[[(then)2Q]sE(condition)E(else)]x

A == B ? C : D;

0AB-^E*C+

_3 # pushes -3

_ # pushes 0...sometimes

c4 5_6  # -6,5,4
c4 5_ 6 # -6,5,4
c4 5_
6       # -6,5,4 # still a negative sign since the next thing it sees is a digit
c4 5_z  #  3,0,4,5 # if it's followed by a non-digit, it's a 0
c4 5_p6 #  6,0,5,4
c4 _*   #  0 # 4*0=0