Use set-param! for paramter W:

This is a workaround to circumvent a "bug" which was introduced with
the advent of the materials library `materials.scm` in Meep v1.6.0.
This library predefines some variables (material constants). We have
naming clash with tungsten (W) and the width of the integration window (W).
Once a vaiable is already defined, define-param can no longer be used.

Airy_2d/Airy2d.ctl

@@ -42,7 +42,7 @@
 (define-param kr_w   0)                     ; beam waist distance to interface (30 to 50 is good if
                                             ; source position coincides with beam waist)
 (define-param M  0)                         ; center of integration window
-(define-param W  4)                         ; width of integration window
+(set-param!   W  4)                         ; width of integration window
 
 (define (Critical n1 n2)                    ; calculates the critical angle in degrees
     (cond