tilde and @ in postscript language

[marc95fr]

Hello,
I try to understand a postscript file, and I find a lot of ~ or @ in the definition.
I never see this in the documentation, what is the meaning of this sign?
For example :
-------- ..... -----
/Xmarc{
/im_save save d/mask? ~ d/maskcolor ~ d
imageprocarray ~ get/s ~ , d/polarity ~ d/smoothflag ~ d +/dx 2 ^ d/dy 1 ^ d +S
/mystring ~ string d/bpc ~ d/ih ~ d/iw ~ d
}b
------- ...... ------
/Ymarc{scol myimagedict @ ` mask?{/polarity
maskcolor 0 get 0 eq{T}{F}? d}if/Width iw d/Height ih d/Decode polarity{[1 0]}{
[0 1]}? d/ImageMatrix[iw 0 0 ih 0 0]d/DataSource s d/BitsPerComponent 1 d
/Interpolate smoothflag d E imagemask}bd

Thanks for help
Marc