@main[]

$s[  ] $s[^s.base64[]]
$a[a]  $a[^a.base64[]]

$b64[${a}${s}${a}] 

'$b64' = '^string:base64[$b64]'

$s[aaa]^s.base64[]=YWFh

I. non-strict:
^check[]
II. strict:
^check[ $.strict(true) ]
II. strict withour padding:
^check[ $.strict(true) $.pad(false) ]

@check[options]

1 ^try-catch{'^string:base64[=YWFh;$options]'}
2 ^try-catch{'^string:base64[Y===WFh;$options]'}
3 ^try-catch{'^string:base64[YW=Fh;$options]'}
4 ^try-catch{'^string:base64[YQ=;$options]'}
5 ^try-catch{'^string:base64[Y;$options]'}
6 ^try-catch{'^string:base64[YQ;$options]'}
7 ^try-catch{'^string:base64[YWE;$options]'}