Small optical change of the erf reporter

* The code of the `erf` reporter has been slightly changed to be a bit more concise.

Author: daimpi

SocNetABM.nlogo

@@ -404,12 +404,13 @@ end
 ; negative domain, therefore the argument (x) must be smaller than 0. For 
 ; sources see infotab.
 to-report erf [x]
-  report exp ( 0 - x ^ 2 - 1.26551223 + 1.00002368 / (1 - .5 * x) 
-  + .37409196 / (1 - .5 * x) ^ 2 + 0.09678418 / (1 - .5 * x) ^ 3 
-  - .18628806 / (1 - .5 * x) ^ 4 + .27886807 / (1 - .5 * x) ^ 5 
-  - 1.13520398 / ( 1 - .5 * x) ^ 6 + 1.48851587 / (1 - .5 * x) ^ 7 
-  - .82215223 / (1 - .5 * x) ^ 8 + .17087277 / (1 - .5 * x) ^ 9) 
-  / (1 - .5 * x) - 1
+  let t (1 - .5 * x)
+  report exp ( 0 - x ^ 2 - 1.26551223 + 1.00002368 / t 
+    + .37409196 / t ^ 2 + 0.09678418 / t ^ 3 
+    - .18628806 / t ^ 4 + .27886807 / t ^ 5 
+    - 1.13520398 / t ^ 6 + 1.48851587 / t ^ 7 
+    - .82215223 / t ^ 8 + .17087277 / t ^ 9) 
+    / t - 1
 end
 @#$#@#$#@
 GRAPHICS-WINDOW